I'm not really sure how you expect this to work -- you are hopefully aware that the spam list isn't a list of URLs, it's just a list of bits of text to reject. I don't think it is feasible to have a whitelist on top of such a filter.
Suppose you configure the spam filter to block only "http" (right here, you already have a problem, if something's not identified as a URL because they wrote hxxp then it still "works". But let's ignore that for now.) Your theoretical whitelist has the single entry "http://wakaba.c3.cx", presumably to allow posting links to this website.
Now suppose someone tries to post the following text:
blah blah blah I am a stupid spammer
http://www.stupidspamwebsite.com/spam/spam/
oh and by the way http://wakaba.c3.cx/
Your blacklist would find that http in the message and try to reject it. A simple text-matching whitelist would find wakaba.c3.cx and say "it's fine" and now someone just completely evaded your spam list.
I don't know of any sane way to do what I think you're asking for.