Ravenology:

Your post seems to contain a typo, as it is asking whether you can convert the thumb shifter to a thumb shifter. I'm assuming, based on the post's title, that you have a stem shifter, and want a thumb shifter? I bought these things on niagaracycle.com for $8/pair, and they kind of rule. got 'em on my 3x9 Long Haul Trucker....

http://www.niagaracycle.com/product_...oducts_id=5146

they have just the right (rear) available ala carte for $5:

http://www.niagaracycle.com/product_...ucts_id=418238

These creatures sort of ratchet; they run as friction shifters, but they "click-click-click-click",although the clicks are non-indexed and don't correspond to any particular gear. Pretty nice piece of metal and plastic, for the money. (Came with cable/housing,too.)

hth
-rob