The normalization routines of DBS use (sometimes slightly modified) SPM methods and you could also choose to start off with normalized images directly (normalize them to MNI space using SPM, FSL, Slicer, whatever works). You just have to rename the normalized files to “ltra.nii” and “lpre_tra” respectively.
You can use the accumbems from the HO atlas, simply isolate it from the atlas file for right and left hemisphere (e.g. Using image calculator) and put the files in a new atlases/subdirectory in /lh and /rh subfolders (or /mixed if you only have one file for lh and rh together). It should then pop up in the toolbox automatically.
Hope this helps!