I'd recommend to keep a copy of the regular client anyway, just in case there is a problem with future updates.
I guess you can just dump the package from the tarball on top of the copied install, but I have not tested this on linux myself.
Neeno's new installer for windows does pretty much the same. (If you don't have a PS install, it can also fetch the missing files from the update server, but that's done by the pse installer, not by pslaunch.)
B.t.w, Neeno has also published a detailed compiling guide for ps-extended on OSX - might be useful also for those who want to compile the official client on that OS.