Is it possible to convert a 5-speed manual Scion TC to a 6-speed?

With enough $$$, anything is possible.

Scion Xb Manual Transmission

2007 Scion Tc Manual

It all depend on the money you are willing to spend off hand I can tell you that about 5 different 6 speed manual transmission can be fitted to about 99% of the vehicles in the road and about 8 will be available withing 2 years both automatic and manual so if you choose either all you need is to contact the manufacturer of any transmission and order it... Just remember that if your vehicles manufacturer does not offer a 6 speed transmission for your particular vehicle that will mean your factory warranty will be void. There is also more to just putting the transmission there has to be programing of various components in your computer, is a nice project so GOOD LUCK..........

I would go with the 6 speed both 5 and 6 are over-drive and since you want to pump up the power and speed you will need the extra gear for mileage. You will find the really isn't any extra shifting with the 6 speed because you wont make it beyond 4 or 5 in city traffic without luggin It.

it would be possible like others said and it would help greatly on highways. the only thing you would really have trouble with is the computer. The real benefit would be when you cruise around 65-70mph on the highway your rpms would be lower so you should get better gas mileage. Since the rpms are at like 3500 it would drop them to probably 2500-3000. I just found this site and ZPI is working on a 6-spd tranny for the tc.

http://www.turbokits.com/tC_turbo_kits.html

it would cost far more than it's worth to do...

youd be better off selling the car and buying a 6 speed. The tC 5speed is fine anyways though so im not really sure why youd be considering this...