Yes, I think the two could mix well togeather. I'd throw out basicaly all the Dragon star gun rules, and use the d20 modern ones.
Gun Prof. Feats:
Personal Firearms = simple weapons
Advanced Firearms = marital weapons
Exotic is still exotic
Automatic weapon rules in d20 modern, along with the Double Tap and Burst fire feats, etc. are great, so I'd keep those.
I like Dragon star Technical Prof. feat as way of seperating low and high tech characters (D20 modern never thought about that) so I'll keep it. I'm not sure how many of the other d20 modern and dragon star feats I'd keep. In d20 modern, two weapon fighting works with guns/range weapons fine, (not in D&D 3.0) so I'd drop the seperate two-gun feats and merg it with normal two-weapon. I'd definatly skip all the d20 modern kung-fu stuff. its all monks now anyway. I do like the dragonstar mobile shot feat, so that would stay... the problem to avoid here is that most D20 clases give out bonus feats ever other level, where D&D does not... so to convert the d20 modern feats over is to assume that the PCs are still geting a lot of feats... which isn't that true. On the other hand all my favorite D20 modern feats were not on most Bonus feat lists. I'd add most weapon realted feats, espeicaly gun ones, to the Fighter bonus feat list... and that clears that right up. most feats in both books were "skill focus combo" feats that gave +2 bonus to a pair of theme linked or similar skills. "alterness" knockoffs really. Also because subdual damage works diffrently in d20 modern, all those knock out feats are useless in D&D and Dragonstar.
as for the d20 modern massive damage rule... no thanks, not for me. sure it works in d20 modern, but d20 modern doesn't have D&D monsters like dragon star does. those suckers can drop more then 12 points of damage all at once easy. (average con score in the groups I run). so in Dragon star setting, I'll use hte d20 modern guns, (better then most bow weapons!) but drop the d20 modern "one shot one kill" massive damage setup.
Skill wise I like the dragonstar meathod. almost everything is the same, and technology is done via the "use device" skill. simple, easy.
The guns in dragon star are to all over the place for my taste. I'd probably take the basic concepts of the d20 modern ones and then bring it over into dragon star style. small hand guns are 2d6, medium ones are 2d8. (blasters are lightning damage, lasers are fire, screamers are sonic, etc). all that kind of stuff. then I'd sit down and make a list of basic guns that fit that mold. Most the ones in d20 modern are designed to have the flavor of the variations between real world weapons, but the D&D and Dragon star simplicity is just fine. You got your light blaster pistol (2d6 damage, more shots on a mini-cell) your heavy blaster pistol (fewer shots 2d8 damage, medium sized) and maybe even an auto-blaster pistol (autofire option, 2d6 damage, medium sized... maybe even 3 shot burst mode!) do the same for all the types. remember screamers (sonic damage) are a little off from the other damage types. I never liked the double ammo for the lead spitters in dragon star (bullets and an energy source means double ammo tracking = more hassel and less fun) so I'd keep lead spitters as self contained ammo/propellant, and just make it a plasma mix etc to make sure it feels high tech and all. I do like that nearly all energy guns use the same mini-cell. I'd also go ahead and make only 4 types of bullets. (small, medium, large, shotgun)
Of cousre I'll use the dragon star price meathod. Don't get me wrong, I love the d20 modern wealth score system, but it doesn't have the D&D feel... but credits and gold and things with prices not price DCs, that's what I want. the new gusn I just created will use prices based of the Dragonstar guns. In a Dragonstar setting I don't have to let "credit cards" and other modern forms of buying stuff without money exist, so the d20 modern wealth system isn't needed really... and I don't want to convert all of dragon start to DCs, and assign bonuses to the standard core classes. Let alone wealth DCs to expensive spell componenets or magic item creation rules!
I'm note sure about the dragon star armors, but I think they'd be ok. the D20 modern ones aren't high tech enough, and D20 modern has an inherent character armor class bonus for classes that I don't want to mess with adding to D&D.
Spells: yah most were converted from D&D to d20 modern. No need to convert 98% of them back to D&D. I think Urban Arcana has some other newer ones that would be worth doing so though.
Advanced classes: I think I'd leave most of them out of Dragonstar. most advanced classes fit niches already occupied by D&D classes. Why have a solider advanced class when there is the fighter core class? also some are based on feature no longer in my "conversion." Example: the feild scientist can "Mcgiuver" just about any device by making Inteligence checks againt the device's pruchase DC. Other classes are based of popularity, and all are ballenced with a Defensive bonus, welath bonus, and reputation bonus built in that D&D and Dragon star doesn't have.
Nope, I'd leave just about all the "advanced classes" out.