I've never used a PSI cartridge kit, but the Berea cartridge kit (with rifle clip) is essentially a modified "cigar" style pen. The tube diameters are identical so I think you could blend a chrome Berea cigar and chrome Berea cartridge kit to get a cigar clip on a cartridge pen. Sorry, I don't have any unfinished cartridge kits to try slipping the cartridge lower ring over the cigar transmission (only obvious potential problem I could see).
In summary, replace upper/transmission portion with one from a cigar kit except use bottom trim ring from the cartridge kit (which you may have to drill).
I did put a cigar top/transmission on a cartridge kit and, while looking goofy (photo attached -- diameter of lower trim ring much too large), it did fit/operate. The cigar top cap is close to the same size as the Berea cartridge cap fittings -- I use two of johnnycnc cigar pen top cap bushings to turn the cartridge caps between centers (IIRC, 0.006" larger than cartridge diameter -- close enough for me!).
I bought the cartridge kit from Ernie at BTW. The cigar top portion is a two-tone chrome/black chrome from woodenpenpro. Then again, your customer may not like the look of the Berea kit and all this is for naught!
Edit to add: The Berea cartridge clip looks like a standard slimline clip (funky adapter at cap) so you could probably substitute a different slimline clip, too. As you've probably guessed, I figure somebody will like the cartridge pen except for the rifle clip someday, too!
