I sometimes feed my audio track outputs to my rackmount outboard gear (comp, multi-effects), then record that output.
Is that what you are talking about?
It's pretty simple, all you have to do is assign the bus and ch's in Sonar (both Aux out and main bus out) so the Aux ch (and only that Aux ch) comes out on a particular Delta chs, then connect that to an outboard gear. Then you record the outboard gear output. I split the output of the outobard via patchbay. One goes to recording input of my Delta, and the other to a mixer so I can hear the outboard in a mix.
Now the hard part is, because of the AD/DA latency, your recorded outboard gear track will be offset by a few mS. This is usually enough to cause combing effect that sounds like a phaser. You usually have to line these up later.
Now, this is more of a hassle than just using a plugin, or recording through the outboard gear. Just so it's clear, I do use plugins, and I try to record the tone I want to begin with.
But then, none of the plugins sound anywhere close to my Lexicon or 1176. Or acidrock's Sansamps for that matter. And sometimes you want to do things to an already recorded track, or something like a SoftSynth generated track.
More ways to be creative is cool.