After adding the monster code from here to the wads and placing a new thing, the monster spawned normally for me.
For some reason, even after adding the code, the monster still wouldn't spawn unless I deleted the thing and placed it again.
There seems to be an issue with the placed thing that prevents the monster from spawning normally.
Which Doom Builder are you using? I tested it in SLADE's map builder.
As for converting the maps to UDMF, from what I know
UDB has ways to do it.