As an open source project maintainer [1] with a set of proprietary add-ons, I totally understand the project maintainer desire to have a single repository with a single version, single build system, and a single place for issues.
I've considered doing the same thing, but it's all the legal complexity that stops me, doesn't seem worth it.
For sure, In my opinion I think this is totally fine as long as it's clearly specified in the likely first point of contact (Generally the readme on GitHub) & license file and as long as the other licenses aren't breached upon clone/download of the source alone, potentially acting as a license trap.
I've considered doing the same thing, but it's all the legal complexity that stops me, doesn't seem worth it.
[1]: https://github.com/beekeeper-studio/beekeeper-studio