Does partnering with institutions that don’t offer graduate degrees meet NSF's broader impact criteria to work with “underrepresented groups”?
I think the reason we want to increase representation of underrepresented groups is that they have the inherent potential for higher representation and achievement and just need some help getting going.

You can't say


  I decided to do recruit theatre majors to our math PhD program, because theatre majors are underrepresented in our program compared to math and science majors.


Theatre majors don't, as such, have inherent potential to do well in math. (Or if they do, some other example will serve to illustrate the point.)

So you'd want to make the case (or have the case already be clear*) that the institution not offering a graduate degree was serving students with inherent potential to do better.

*Regarding that, see @ElizabethHenning's comment below.