Is there an encryption extension or tool for Nemo in the Ubuntu repository ?
Do you mean something like this?
More like the following. I was hoping for something from the official repo .
nemo-seahorse is an extension for Nemo which allows encryption
and decryption of OpenPGP files using GnuPG.
nemo-seahorse is a fork of seahorse-nautilus 3.3.1.
If you are referring to the Universe repository - nemo and related stuff are maintained by the debian cinnamon team. Probably the best way to request a new cinnamon package such as nemo-seahorse is via their mailing list firstname.lastname@example.org