Tips are pretty easy to make on a lathe using the following method.  You could also do this without a lathe if you get creative using hack saw / files and a cordless drill:

1. Cut off end of soldering tip (tip shown is Weller CT6F7, good idea to buy a few spares) using lathe or hack saw.

2. Drill and tap a 1/4-20 (or similar) thread in the end

3. Use a die to make the matching male thread on a piece of brass rod, then turn a shoulder that matches the inner diameter of the threaded insert you want to install.  The shoulder can be created on a lathe or by putting the brass rod into a cordless drill, running at full speed and filing the shoulder down until it fits in the insert.  The brass tips are now interchangeable and will withstand pressing forces better than if you just put a shoulder on the copper soldering tip.