You can use 3.5 adventures really easy. All you really need to do is give a monster its CMB and CMD, and you only need to do that if a player is using combat maneuvers against it. Changing its HD and other little things is really unnecessary. You can always just add 1 or 2 HP per HD to the creature if you feel its HPs are too low.
Skills are pretty easy too. Just use the highest of the creature's Spot or Listen for Perception, highest of its Move Silently and Hide for Stealth, etc. The only thing you really need to be mindful of is that some creatures that used to be immune to sneak attacks no longer are, such as undead, I believe.