Basically how and what should I convert to change the Psion from the Expanded Psionics Handbook for 3.5 to be usable in Pathfinder?
Psionics have now been converted to Pathfinder rules officially via Psionics Unleashed from Dreamscarred Press. It was open play tested prior to publication so the rules seem pretty solid. I've been running a Psion (started at 1st at 5th now) and haven't run into anything weird.
It might be worth noting now that Dreamscarred has continued to support psionics with many additional releases. I got my Psion up to level 15. He was quite powerful in terms of ability to lay out raw damage. Can't speak so much for utility powers as I designed him as essentially an elemental energy carpet bomber.
It looks as though there are rules for Psionics on the Pathfinder SRD. I should note, however, that the content appears to be from a third-party publisher, so your mileage may vary.
I'd provisionally just convert skills and the like; I don't think the Psion particularly needs additional powers, since it's not like it was especially weak in 3.5. If you think it does, a bonus feat progress is probably simplest. Depending on power selection, you may need to rebalance the powers themselves some, since a whole lot of spells got edited in Pathfinder.