Hi,
You can use:
fiber -seq=AAAAAAAAAAAA 12bpADE.pdb
To generate a helix of 12 base-pairs
or
fiber -seq=AAAAAAAAAAAA -single 12bpssADE.pdb
To generate a single strand with 12 Adenosine bases.
You can substitute with any nucleotide letter you want.
fiber -seq=ACTGTGCTGCTG my12bp.pdb
In general the fiber program (which is included in X3DNA) is what you're looking for.