Thanks for the welcome!
I distinctly remember Ponder Stibbons worrying about the fact that new components seemed to appear overnight. Can't remember which book, maybe The Hogfather... So, no, the wizards don't add things (if they would, the first candidate would be Ponder)
Edit: Found it, though it's a bit different from how I remembered it
"Every time he came in here, it seemed to Ricully, something more had been done to the... engine, or thinking machine, or whatever it was. Sometimes stuff turned up overnight. Occasionally, according to Stibbons, Hex hims-- it
self would draw plans for the extra bits that he -- it
Seems to me that sometimes HEX asks the wizards, but sometimes takes care of it hims-- itself.