Although intensive,
Those who are willing to become a maintainer for a DE, seem to be able to put forward a package addition request on the github:
https://github.com/getsolus/packages/issues/6979
^ Cinnamon proposed by androidnisse who ticked the box to maintain it, notes mention it would need further discussions as it affects ISO's. and Docs.
I don't know how intertwined the teams are between the two Distros 🤷
Technically though for most packages/themes you can do a package addition request: https://github.com/getsolus/packages/issues by creating an issue here and using the appropriate tags.
and with a bit of time you'll get a response or see if someone is interested in doing it who is capable/has the time/overlays with what solus team is comfertable with.
I imagine a new DE into the Repo's is a very big change, since its not just having it there and it as a try at your own risk, but it would need to be tested thoroughly to ensure it worked on solus/crafted in such a way.