If your transmitter does not have the mixing available to mix two different channels into the elevator channel you will end up with only two options. One is to purchase a secondary mixing device such as a JR Matchbox or a similar unit by Futaba. There's one more company that makes these things (the actual inventor, I think) but I can't rember their name. The other option is to run the linkage up on one servo and rotate the servo on the other side 180 degrees and run the sevo arm off the bottom if necessary. That method rarely works out well with large surface travels.