The long answer is the the forum, as it has been asked before.
The short answer, is that there is no simple way due to the differences between Elastix & Trixbox.
You can experiment with bringing your Trixbox Freepbx upto the latest version and performing a backup and try restoring it on an Elastix machine. I have done this several times before and unless you are doing 200+ extensions, it is not worth it due to the manual checking & correction which is required. Other reasons why this is not a good idea is:
1) The change from Zaptel to Dahdi (will definitely cause you some issues)
2) the file and directory structure of Freepbx on both ELastix and Trixbox differs
3) It is my understanding that Trixbox forked Freepbx, with changes.
4) many other hidden issues that may not surface for a while.
Almost every trixbox to Elastix upgrade that we have performed is by taking screen shots of every screen. CaptureWizPro is a good utility for this allowing you take a complete screen by screen configuration in less than 30 mins.
Doing it this way, you know that you have a clean system and clean config.