The 10 shot magazines in the ruger P series are not pinned. The have a large base that goes up into the magazine. I don't know if the base can be replaced with the base from a 15 shot magazine. If you go the parts list for your pistol at ruger.com you will see the different bases for the magazines. If you have had your Ruger for a while a new magazine spring won't hurt a thing and they are under $3. Numrich has mec gar magazines for the ruger P series pistols for around thirty dollars. Numrich also carries the original ruger magazines.
You can not convert the magazine in the picture to a 15 shot magazine. The big plastic piece at the bottom is to make up for the length that a 15 shot magazine would have. Mec Gar makes a 17 shot magazine for the P95 that fits flush with the bottom of the grip. I would try to borrow a 15 shot P95 magazine to make sure it won't hang out of the bottom of you pistol.
Promag makes a 32 shot magazine for the P95. Once I got my 32 shot magazine broke in it is just as reliable as my stainless steel magazines that came with my gun. The 32 shot magazine is a blast when plinking and the extra weight in the handle is handy when shooting IPDA. I have yet to have anyone complain about the 32 shot magazine as long as I only put 10 shells in it.