The original seat attachment part is held in with an e-clip in the original fender. Not welded in. If your fender extension is Milwaukee Bagger it's probably the part pictured below or one of the close cousins they offer.
If so I've dealt with one of those. It was held on with 3M two sided tape. If yours is the same as what I was the fender extension stands just a little proud of the fender. The threaded insert in the original fender may or may not still be there. Even if it is I'd worry the normal seat bolt may not reach. Don't know for sure but would want to look closely.
If the seat bolt does read and the threaded insert is still in place, as has been mentioned above, you'll have to carefully make a hole that lines up with the original threaded insert. If the insert is gone you could drill through the original hole (with the tire out of the way) to align the hole but the painted fender extension may make it difficult if not impossible to install the threaded insert normally as you won't be able to get at the hole to snap on the e-clip.
Look for that original hole. Try to see it. If it's there and you can take a skinny punch up through the middle of the threaded hole and put a divot in the fender extension. Or grind a point on the end of a 1/4" bolt and run it up from the bottom side. Let that point put a little divot in the fender for you. Find your mark on the top, use a light blunter punch from the top and knock a divot back the other direction, and drill there.
Hopefully you can see and use the original hole in the original fender. And, hopefully, it's not too much of a pain in the ass to get to.
Honestly I'd almost hope the threaded insert was gone and I had no original hole to guide me though. Might be easier to just set the pillion in place, see where its rear bolt hole lines up, make a divot and drill a hole, but drill a hole big enough for a nutsert. Then just install the nutsert and, voila, threaded hole!