I bought a frying pan and made it non-stick on my own. Does it need Tevillah?
Yes it does. Since the frying pan is made of metal and was brought from a gentile requires that it be toveled with a bracha. The fact that a Jew added the non stick coating to the pan is not going to remove the original obligation for the pan to be toveled.
Y:D 120-1. Although there is a halacha that if a pot got a hole in it and a gentile fixed it, that he also has a partnership in the pot, that is because the pot wasn’t usable then and the gentile in a way help recreate the pot, but in this case the pan was perfectly useable beforehand. Therefore it needs regular tevila with a bracha.