Epiphany web extensions

I’d like to use bitwarden in Epiphany. How can I do this using home manager? (unstable branch)

There is no specific support for this. You would need to look at Epiphany source how extensions are installed and then replicate it in Nix, e.g. using home.file Home Manager option.

That’s what I thought. Is there a way I can raise a github issue to add this?

Home Manager has its own issue tracker. But I would say your chances of someone implementing it are pretty low – features are typically implemented by people that want to use them, and the pool of Epiphany users is pretty small.