Convert fcl::ocTree to FCL mesh - Merge 2 or more FCL mesh to one model

How I can take fcl::ocTree and convert it to FCL mesh?
There is a function in fcl test call: generateBoxesFromOctomapMesh
It generate vector of CollisionObject from the octomap.
Inside the function it create boxses from fcl::ocTree, and convert each box to FCL mesh and convert it to CollisionObject.
Besicly I need to mearge all the models to one big model. How I can do it?

Thanks for the help, Tal.