The OP was more referring to Mercurial, hence we have renamed this topic and created a new one for Git:

http://smartgit.userecho.com/topics/15-git-stash-selected-files/

I've added a new topic for the Log -- please vote for it:

https://smartgit.userecho.com/topics/16-/

So your request is just to add the Stash menu item to the popup menu? Fine. Regarding stashing of a subset of files, there is already another topic:

http://smartgit.userecho.com/topics/5-stash-selected-files/

Actually, this is already in our issue tracker (SG-7617 and SG-9945); I've added a new topic here:

https://smartgit.userecho.com/topics/12-/

How will that be different to the File table filter?

You are aware, that Stash does not work on files, but only on the entire repository?

In the Preferences, section "Tools" you can already configure tools. Isn't this what you are looking for?