Is it possible in SASS to inherit from a class in another file?
YES! its possible.
If you want all <button> elements to inherit the .btn class from Twitter Bootstrap's Default buttons
In your styles.scss file you would have to first import _bootstrap.scss:
@import "_bootstrap.scss";
Then below the import:
button { @extend .btn; }
**I might be mistaken, but if I get what you're trying to do, can't you just use the @extend .classname; command inside the element that you'd want to extend? Naturally, you should only modify your own code to preserve updatability.