I wouldn't know why you'd want to do so(slowing down the search), but it seems possible (or close enough).

Scroll at the way to "Make Spotlight index source code" here. http://www.macosxtips.co.uk/index_files/terminal-commands-fo...

That assumes you care about the speed more than the results. Quicksilver or Alfred completely replace Spotlight for anything frequent; I'd be happy if it took 5 minutes, if it could always find what I was looking for.

The command line version of it (mdfind) seems to search the entire file system.