Hi J,
I use Plastimatch for something similar, the files I have are in mha format but it sounds like it would work for your data so would be worth a try. I do it in 2 steps:
# add structures together into one nii file with different weights for each structure
plastimatch add --output niftySS.nii --weight "1 2 4 8 16" bladder.mha rectum.mha r_fem_head.mha l_fem_head.mha bowel.mha ;
# convert .nii file to DICOM
plastimatch convert --input-ss-img nifty_SS.nii --referenced-ct . --input-ss-list list.txt --output-dicom DicomSS ;
list.txt has the values and colour codes for the structures:
0|255 0 255|bladder
1|255 127 0|rectum
2|135 206 235|r_fem_head
3|128 128 192|l_fem_head
4|239 193 7|bowel
Cheers,
Tim