Visit Jeff Davis' site. http://www.homecnc.info/
He talks about the conversion of his mill drill. I think his is a larger model but the principles are the same. It will at least give you a starting place.
I have a Busy Bee Mill/drill (Rong Fu knock-off) also, and would like to purchase a simple bolt on kit for it as my time is limited and can not afford to build from plans like Jeff's Plans from home cnc.
I am going to email Jeff and ask about selling pre-fab'd kits???
The only other company I have come across is CNCMasters.