Can I add a breaker to my main disconnect panel?
First of all, congrats, you did a great job researching before asking the question. That's expected of participants here and you did that.
To answer your question, yes, there shouldn't be any reason you can't add a breaker to the outdoor combi box. Others here might chime in with local code issues, but I doubt it. If it makes you more comfortable, turn off the main breaker to shut off power to the buss.  Remove the cover, clip the new breaker in place, twist out the cover for that space and wire up the outdoor circuit using conduit and THHN/THWN or UF (UF is hard to work with!), NM (Romex) isn't legal in outdoor installations, even if in conduit.
I don't want to be a dick here, but this is a pretty basic question indicating a basic unfamiliarity with simple wiring. While this is a DIY Home Improvement site, remember that improperly installed electrical circuits can be dangerous.   You might want to grab a book at HD or Lowes on basic wiring principles. Again, I really don't want to be rude or condescending, you might actually enjoy reading about basic wiring and how to do it right. Take care and stay safe.