has anyone tried using a sleeping bag and just converted it?
Yes. The trick is getting the bag to stay against the hammock bottom - you ususally need to tie or shockcord it (stretch cord) so it stays against the hammock bottom - if it droops you get cold.
Here's my hammock last weekend using a pad inside, an integral designs silponcho on the outside, and an exped wallcreeper primaloft sleeping bag held onto the bottom of the hammock as an underquilt - wasn't that cold out, so I was pretty toasty.
I had the poncho there, but I also had the bag tied to the hammock tieouts on the end with shockcord.