I have a tokina 500mm f8 lens with a P/K mount

YOU CAN BUY A T OR T2 ADAPTOR ON EBAY

This will depend on the lens you have now.

Some Tokina lenses have a universal fitting, with a separate adaptor mount for each camera body they fit. Its usually the last 5 - 10mm of the mount, and its identifiable by a second release button, which allows the specific fitting to be changed. If you have one of those, you just need the Nikon fitting.

If the P/K mount is an integral part of the lens, you have a couple of choices.
(1) You could get an engineering shop to alter the mount. This costs a lot and probably isn't worth the hassle. (It might be better to sell your lens and buy the nikon equivalent).
(2) You could use an adaptor. These are readily available on eBay.
For example:
http://cgi.ebay.co.uk/Pentax-PK-K-Lens-to-Nikon-AI-AF-F-camera-mount-Adapter_W0QQitemZ150400359906QQcmdZViewItemQQptZUK_Photography_CameraLenses_Lens_caps_hoods_adaptors_ET?hash=item23048f5de2

Note that just because it has the right (i.e. Nikon) fitting, it doesn't mean that it will necessarily have full functionality on the body you intend to use with it.

Some Nikon bodies are "extremely fussy" about which lenses they can use, and it might be worth evaluating whether your proposed solution would work with the camera body you have in mind before committing to any sort of expenditure.