Where are you located? In North America, the MX-800 uses 418MHz for RF while X-10's RF devices use 310MHz so there is no way for them to communicate directly.
Even if they used the same RF frequency, the X-10 RF protocol is very different from their IR protocol. The X-10 codes in the databases of most remotes are the IR codes for use with X-10's IR543 which receives IR and sends PLC (Powerline Control). There are links to documentation for the various protocols at...
[Link: embed-x.com] Outside of North America, X-10's RF devices use 433.92MHz. If there is a European version of the MX-800 that uses 433.92MHz it may be possible to learn the RF codes and then send them directly using techiques similar to those for ProntoPro explained at...
[Link: mbx-usa.com]