I've thought about something quite similar before and my thoughts on the subject are that there are 2 options:
You character models are created in parts one for torso, waist, head, upper arm, lower arm etc...
You model the armour in the same parts with similar dimensions. I think the vital thing is to ensure that 0,0,0 is the in same place in all models for a specific body part.
Then in your software you build up your character from these parts using the LIMB commands.
Once you've done that it should be fairly simple to replace a default body part with an armour/clothing part or whatever using the DBP command, I think its replace limb or something like that.
The second way I saw of doing this involves making the model completely then using the command attach object to limb to attach the armour to the body model at runtime when necessary...