To use an external obj:

1. create a directory in the geometries folder (use a recognizable name, such as your alias).
2. copy the obj file in that directory.
3. edit the pp2 file: you need to replace the 'prop' section containing the 'geomCustom' tag and the geometry definition (lots of numbers) by a new 'prop' section looking like this:

prop propName
{
storageOffset 0 0.3487 0
objFileGeom 0 0 :Runtime:Geometries:yourDir:propObjName.obj
}

The propName tag must be the same as for the second 'prop' section containing the various 'channels' (ie.dials) definitions.

This is the process I go through for my own items and it seems to be working fine.
I think there may be a tutorial about this somewhere in the Tutorial section here.