Yes, that's certainly possible, although you should take care to have at least one of the nanotubes metallic, since electrodes normally need to be conducting. That is, choose a (n,n) nanotube for the electrodes.
The first thing to consider is if you want the tube to be capped or not. If you do, you should start by following the
tutorial on this - or if you use 12.8 (beta-version is available) you can Add>From Files some existing capped tubes.
After that the procedure is very similar to
http://quantumwise.com/publications/tutorials/builder/172, except the "alignment atoms" should be initially inserted as the center point of the hexagon/pentagon on the tip of the capped tube (my suggestion, you can choose differently if you prefer) and then moved out in Z.