Stonefish,

The easiest way to add a fighting butt to a fly rod is to buy a kid's bicycle handle grip and just slide it on over the end of the reel seat. It's only there when you need it.

Otherwise it depends on the reel seat. Some have a press fit aluminum dowel that is expoxied in, but those should be removable with a little heat and prying. The end of the blank is usually right there, but naturally, there are exceptions.

There is another alternative to a threaded butt cap. If the end of the blank is visible when you remove the butt cap, use a piece of discarded rod blank sanded to fit, or even sand a wooden dowel to fit about 3" into the butt of the rod. Glue cork to the exposed part, and turn to desired shape and diameter and add a new butt cap and you're done.

Sg