Ok, I understand… If you had a Windows PC this may be possible. I know some users have had success manually typing in UNC paths in the “source data” section. It can work if the user context under which Duplicati runs has the same credentials on the NAS (or it allows Guest access).
This approach doesn’t work on Linux as far as I know, and may also not work on Macs. (I have no experience running Duplicati on a Mac.)