Search in Smart Group?

I’ve just been come up with a useful smart group (posted in another thread), that allows me to see only the skim notes in my bookends attachment folder (without the pdfs to which those notes refer).
Now, if I want to search for something in that search group, it appears that I cannot do this directly, since I don’t have the option to search in a smart group.
Nor can I search for .skim files, since .skim is not in the popup list for Kind.
So I have to do my search, and then sort by kind. There I find some .skim files.
Is there any way to shorten this process (i.e. by searching in a smart group, or by having .skim show up in the Kind popup list?



Unlike normal groups, smart groups don’t hold documents. They are merely search sets that display the list of results when opened. Since they are empty of document content, they would return null result when selected for search.

In that case, however, is there a way to only search .skim files?
I see that .skim is not on part of the Kind popup. But perhaps there is some other way?