Is it possible to convert a Lexus IS 300 transmission to manual?

yes, but no

technically it can be done.. but it would be very expensive.

It's not just replacing the tranny, you'll need a new starter,ignition, differential ... the list goes on.

It's cheaper to trade in your car for a manual.

PS: Why? As automatics go, the is300 has a great one. If your boosting performance you could upgrade your tranny ( or parts) to handle the increase strain. If you just want the control, get used to the paddles (buttons, with some practice they're very useful - try going into a sharp turn at 60, keep your foot on the gas, down shift , and power through)

or

trade in for a stick.