After Effects - Add 3D element to another 3D project

There is nothing keeping you from copying layers and pasting them in another comp. There is nothing keeping you from putting a bunch of 3D layers in a separate comp, then nesting that comp in another comp, and collapsing transformations. 

If you get too complex you can easily run into the limitations of the C4D rendering engine. 

Another option is to use Shape layers and text layers in a Classic 3D environment, then save the comp as a Maxon C4D file, open up C4D lite, and do your extruding and bending there. I do that all the time and it's pretty easy to save better-made and more versatile 3D files using C4D lite. I almost never use the C4D rendering engine and extrude text or shape layers or bend other layers. It's just too clunky for most of my compositing work. On rare occasions, I'll use the workspace to extrude something simple for a motion graphic, but I can't think of the last composite I did that was not all done in the Classic 3D workspace.

Nothing to save, because there is no persistent geometry there. All stuff created using the basic extrude functions is generated at run time. If you want reusable geometries you have to put up with actualyl modelling them in Cinema 4D and importing them via Cineware. Otherwise you are limited to the good old importing of projects into other projects and then possibly copying over individual layers from comp to comp with all the caveats that has like an increased risk of crashes or the whole thing not working at all. That's just how it is. Even after years of Cineware and the C4D renderer AE is basically garbage for this kind of work.

Mylenium