You could save a file with geometry which you want to re-use as a template and re-open it.
Alternately, you can merge two .c2d files — EDIT: while there was a utility for this as noted in: Ruler in Carbide create - #33 by ClayJar
As @ebr and @GJM have noted, since this thread was initially posted, Carbide Create has gained the ability to export SVG files, and to import .c2d files.