I make some full tang knives with a butt cap. There are a number of ways to accomplish this. With thick steel. I often just drill and tap a hole in the end of the tang and tread a piece of all-thread rod into it. With thinner steel, I sometimes just cut the blank with an extension that I will thread later. Other times I cut a slot in the tang and sometimes weld the rod into the slot.
Here is a simple method that required no welding but will provide a secure method of attaching a butt cap.
Start by drilling a hole the diameter of the rod you will be using. Then, cut a tapered slot to meet the hole. It is important to taper the slot so end up with a keyhole type slot to help lock the threaded rod into the slot. I often use 10-24 all-thread for steel around 3/16" thick. You want the narrow part of the slot to be slightly thinner than the threaded rod.
Next, just tap the threaded rod into the slot. This should shear off the threads at the narrow part of the slot.
Grind it flat to the end of the tang.
You can see how the tapered slot has locked the rod into place so it will not pull out from the end. The handle scales with lock it in place from the sides.
Now just cut a piece of bar stock to size, drill it, tap it, and screw it onto the rod. I use epoxy lock it in place and will usually add a colored spacer for accent.
Shape it to the handle and sand everything down (or polish) and you're done.