There are a number of possible methods, depending on the type of case and its size. I haven't actually done it yet, but want (I know... I just love old things that go bang!) to, and have looked into it.
Most methods involve priming with a no 11 percussion cap. One is to drill a hole transversely through the case head for the pin, and to enlarge the hole on one side to accept the percussion cap primer; you may need to fiddle with pin hole to make it large enough for the flash to reach the charge.
Another is to turn up and drill a battery cup arrangement to replace a normal 209 primer in a shotgun case which will hold a cap, pin through the case head as before.
In all cases, a lathe may be your friend, as may a good drill press and a selection of bits.
Keep in mind that pinfire rims (at least in shotguns) were smaller than for those new fangled central fire saftey cartridges by that Daw bloke! I have saved posts for this from elsewhere if you want me to try and link to them.
RG
