Hi Andy - for the joints you wish to lock into position, you can put a Rigid Transform (parameterized to the same rotational/translation displacement as the degree of freedom) in parallel with any joint you wish to have locked and then comment out that joint.
If you are exporting from CAD and all parts have the same reference frame (this is common if you export a set of parts all at once as STEP files) you can just lock all parts to the same reference.
--Steve
