You can, but a lot won't take the work, particularly if the conversion is incomplete or missing parts. They typically won't source something like this for you either because the parts are scarce at wreckers and it's time consuming to do (and the resulting inflated prices for these components mean you will probably say it's too expensive in the end). None of the V6 stuff will work with the V8 if you were thinking along those lines. It being a turn key kind of deal where you drop it off as an automatic and pick it up as a manual and the mechanic does everything is not likely.
They do exist, just nobody is selling much ATM.