Title is pretty self explanitory. I have a MX-900 universal remote. I also have a ceiling fan that came with a remote control that I believe is RF. anyway I would like to be able to dim the light and control the fan speed (both can be done with the fans included remote). Just wondering what it would take to make this happen.

The MX-900's RF can only be used to communicate to its own RF base station. It can't control RF-only equipment. Just about every other IR/RF universal remote is the same, one exception being the Harmony 890 which can also control Z-wave equipment.

Is there a way for me to control X10 modules with it? I think I read about an X10 fan control somewhere.

You can control X-10 by using an IR543 IR-to-X10 converter. They're about $20-25.