good luck. the magwell is easy. but you need the plastic mags. and last i looked they are hi-cap mags only and having plastic mags pinned,,,well i dont think is is possible. go read the hkforums, at hkpro.com as there are some states that dont allow hicaps, and someone there might give you an idea on what mags to use, ..
what your looking at is the "jet funnel" they are easy to put on, and to take off, but its the mags, normal usp mags dont fit for obvious reasons. but i havent looked into this ,to really know.
good luck, and let us know here what you find out. because im sure your not the only person interested in it.
ps, just so you better understand when i say plastic mags,, take a look here
h ttp://www.calssportingarmory.com/MAG_HK_USP_FS_9MM_18RD_NEED_JF_p/mghk219612.htm
to better understand what i mean by plastic, they are hicap , that hold 18rd for 9mm and a few less for .40
thats why i say, having them pinned might be a little hard.