Hey!
I am trying to do something similar but I am facing some issues. I made a post in the Forum earlier today.
Here is the link, perhaps you can take a look or get some answers too!
http://official-rtab-map-forum.206.s1.nabble.com/Combine-3D-Lidar-and-RGB-D-Camera-to-create-a-single-3D-point-cloud-sensor-fusion-td10162.html