Construct a self-define simulation cell

Hi,
I want to create a cubic box (for example 20 * 20 * 20 Angstrom^3) with a CNT with specified length and diameter in its middle in materials studio.

Is that possible in materials studio? I try this, but I don't know how to transfer CNT in the center:
1- build - build nanostructure- single-walled CNT - periodic
2- build - symmetry- supercell - increase C (z direction)
3- build - symmetry- lattice parameter - change a,b to desire value- change gamma to 90 
(attach file)

 Now, how can I transfer the CNT to the Center of the box?

Thanks,
Karan