The rod-shaped tangs can be threaded to take a decorative pommel nut; do a web search for knifemaking supplies. Round balls (over a pommel) are common for dirks. an older style was to peen the tang over a square nut. With either method, you might need to draw the temper from that end of the tang beforehand. Setting the tang in the hilt with epoxy isn't bad insurance. Sgian dubh's can simply be epoxied into place with the tang in a blind hole; it doesn't go all the way through to the pommel; so no nut needed. If you can thread it, the teeth will help bind it to the epoxy.
Flat tangs are often meant to have 2-peice hilts, called scales, applied with rivets. If the tang doesn't show, it's set into rabbets cut in the scales before they're fixed to the tang. Again, epoxy isn't a bad idea.