You can't use Reason to control external devices, unfortunately, so you won't be able to sequence it in Reason while it's ReWired to Sonar.
One option you have is to sample in Sonar the bass sound you want to work with then use those samples with the NN19 or NNXT. I do this myself sometimes when I just want to focus on Reason.
But don't let ReWire put you off, it's fairly invisible when you get the hang of it. Generally, you just load your ReWire host first (in your case, Sonar) and then the ReWire slave (Reason). But I haven't used Sonar at all, so I can't give you more specific advice; you might find something in this tutorial from Cakewalk.
- alex23 -
"Then I saw her face and I got this weird, deep sickly churning in my stomach; she looked exactly like George W. and I realised 'Phark, it is her'. George W's daughter going nuts to We Want Your Soul, miming the words." [Adam Freeland]