As Daniel says the JV/XP synths are all ROM based with no RAM based sample support. ROM expansion is the only way to get new samples to use, and fortunately the JV/XP expansion card library is a pretty healthy selection of boards.

I very much doubt whether you would even find 3rd party products that do this, as the JV/XP samples aren't linear samples, but are a proprietary compressed PCM format (compression is around 2:1) that Roland does not release, so 16 bit linear samples will just not work even if you could kluge some sort of RAM board onto it.

Bottom line is if you need to sample, you better get a sampler.