watching scss files from two themes

I have a custom Zen subtheme in sites/all/themes/mytheme, and I'm running compass watch to keep my css files up to date. So far so good.

I'm using seven as my admin theme, and need to add a few things to its css. I'd like to add a scss file and have it generate a corresponding css file. There's gotta be a way to do this with #add_import_path, but I haven't figured out how yet. I tried

add_import_path "../../../../themes/seven/scss"

but it isn't recognizing it.

Also, I noted that the seven theme doesn't use a css subdirectory; I tried adding scss and css subdirectories, hoping that would help.

Since it's not a best practice to directly hack a base theme like this, and seven doesn't provide subtheming, should I maybe make a copy of it under sites/all/themes?