if you want to hook up the 3 wire DC fan to the 2 wire converter then just ignore the third wire on the fan, its usually either blue yellow or white and is used for RPM monitoring
Does it have to be a DC fan? When I DIY'd my hood, I was originally going to go with a comp fan & power supply ( mostly cause I'm a cheapskate ). Radio shack had a really nice AC fan on sale at the time for only $20 Cdn. that worked perfectly.