Hi,
PlayOnLinux does not manage Wine "prefixes" it didn't create itself, and there's no supported way to move existing prefixes inside PlayOnLinux, but it shouldn't be too difficult to do manually either.
If you already have some PlayOnLinux virtual drives already, or create one manually using Configure button > New button (at the bottom left of Configure window), you can see that subdirectories below ~/.PlayOnLinux/wineprefix/ are basically Wine prefixes, plus one playonlinux.cfg that contains at least informations like Wine version and architecture to use, and a playonlinux.log file that's entirely optional.
So you can, say, create a virtual drive in PlayOnLinux then replace all but the playonlinux.cfg and playonlinux.log with the registry files, dosdevices and drive_c directories from an existing prefix, and be good to go...