What about the Collect Signatures workflow? If you need to have someone outside the company sign the file, you can include them on the Collect Signatures workflow. They will then simply send a copy of the file back with their signature. However, you can’t merge that signature with the other signatures in the document library, doing so might invalidate the other signatures that are already there.
In this case, the best thing to do is to upload their separate version into a separate folder called External files.