This is simplicity itself. Once you fermentation is over, simply transfer it into a plastic barrel and put some CO2 onto it. This can be done via:
Leave your barrel for a couple of weeks, at this point either transfer into it's final barrel (or syphon to a fermenter, clean the trub out of the barrel and syphon it back) or if you are going to put the barrel in a place it won't be moved from during serving, simply move the barrel to this location.
While plastic barrels are great for storing your beer during conditioning, preventing air getting in and spoiling the beer, I would recommend investing in a cornelius keg to dispense your beer, in which case go for a widget world injector and look at our Cornelius Keg page for more information.