I'm mystified right back.
Conversion is required in the sense of flipping the saves of monsters and NPCs into defenses, and adjusting the Reflex defense to account for armor and natural armor. I guess you'd have to remember that Hide and Move Silently are both opposed by Perception, too.
Finally, you'd have to address one or two special qualities or special abilities. You
need a ruling for Incorporeality because Saga doesn't have one (but 4e almost certainly will), and you'd need to decide to either use status effects as-written (no conversion required) or convert them to the damage save system.
Beyond that (which can easily be done on the fly, or in a few minutes at home), what issues do you think would crop up with running Saga PCs and rules through a D&D adventure? I seriously see no reason why, with absolutely no pre-adventure prep to account for the rules differences, I couldn't run my party of Jedi, Scoundrels, Soldiers, Scouts and Nobles through Expedition to Castle Ravenloft or The Red Hand of Doom or the Savage Tide adventure path. The result might be ludicrous in the sense of having guys with blaster rifles in a D&D setting, but it would not be unplayable due to the rules.