This requires a modified version of ROOT's geocad library.
You can find it at https://github.com/whit2333/root/tree/geocad_improvements

Features:

- 'npdet_to_step list compact.xml'
   This will print names of level 1 volumes. This is useful for the part
   selection.

- 'npdet_to_step part -l 3 EcalBarrel -o EcalBarrel.stp compact.xml'
  This will select only the EcalBarrel geometry and output a new STEP
  file. The Geomtry will only be output up to and include level 3.

- These commands can be added together:
  'npdet_to_step part -l 3 EcalBarrel \
                 part -l 2 HcalBarrel \
                 part -l 4 SiVertexEndcap \
                 -o det_parts.stp  compact.xml'

- See 'npdet_to_step help'