Hi. I'm working on a mini-knowledge base in SilverStripe myself, so thought I'd reply to you. First question is... what exactly is it that you think SilverStripe can't do? Obviously it can search for those phrases, but presumably you mean you'd want the results of searches for those phrases to include results where the article included the expanded phrase. Am I right?
If so, a couple of options spring to mind. One would be to have a separate keywords field for each article in your knowledge base, which is included in searches. Another would be to have a database of acronyms and their expansions that you could use either to expand acronyms just before you do the search, or contract (acronymise?) when you're adding articles. The database could be just a text file or hardcoded in PHP, or better still, included in the MySQL database, and managed using the new ModelAdmin model.
That all might be way more complicated than you're envisaging. Of course, even Google doesn't do clever expansions like that... Wikipedia does in a way though, by having pages that redirect from acronyms to their expansions, and probably vica-versa.
For what it's worth, I spent a couple of hours looking at the Interspire Knowledge Manager documentation last night, and though ooh, you could do a lot of that in SilverStripe.
Hope this waffle helps!