No. you can have multiple uv maps for an object. getting the template from that mess can be a pain.

Save JUST the new parts, JUST for the prupose of getting thier template easily, and make sure they all have new materials.

the Uvs will be the same in the original obj with the additions, as the exported additions only.

You coul ddick around in uv mapper to hide everything else and then make a template, but why bother if there is a simpler method?

Way too many people take way too many things way too seriously.