Saga rules don't port well into standard d20.
Standard d20 rules port extremely well into Saga, with occasional conversions and glitches - the latter usually caused by flaws in the particular standard d20 rules. Saga is like a significantly upgraded but still backwards compatible computer OS; it doesn't handle all the software for the previous OS, but between inline support and a bit of emulation, it handles most of it. The old OS, however, can't really run any of the new OS's material without it being rewritten.
To use most standard d20 material in Saga, you really only have to do one thing: convert the saves to defenses. For monsters, this means adding 10 to their saves and then adding AC bonuses like natural armor to Reflex. The latter sometimes takes a bit of tweaking. For classes, it means assigning a suitable starting bonus (+0 to +3) to each save - you'll grasp it quickly.
Also, when dealing with special abilities like Incorporeal, you'll have to make rulings.
Once you've done that, you can drop firearms from d20 Modern, classes from both d20 Modern and D&D, monsters from anywhere, mecha from d20 Mecha (or d20 Future

) - even Vancian or power point spellcasting or psionics, if for whatever unfathomable reason you wanted them.
The conversion, at least upwards, is much, much smoother than to True20, which requires almost a complete rebuild in most cases.