We have received a signed document in the post - is it possible to upload this signed copy on the system?

Yes it is; firstly go to the entity to which the document is attached, so Client, Supplier, Candidate or Assignment, and there you will be able to select/edit the document:

Upload a copy of the signed document and click Update to save it:

Next click the Manage Workflow icon to update the workflow (as the system is still expecting the document to be signed electronically):

Then click the Set as Signed icon here to update the workflow:

The document will now be green ticked as signed with the manually signed copy attached.