I'm no expert, but the only way I can think of doing this is to use a couple of custom search engine entries in Options > Search Engines.
I tend to look (initially) for HDTV Xvid files and my (current) preferred search engine is Kickass so I've created a custom entry in "Search Engines" that looks like this: -
Which appends xvid and hdtv to the search for me.
I know Kickass has "OR" functionality so something like: -
would only find stuff containing 1080p OR 720p
It may take some fiddling around to get the syntax right for a particular torrent search engine but it works...