More specifically a dbx 266xl and a Focusrite 6i6? I had an older mixer and used its 'insert' input, and I think the side chain output but no such animal on the 6i6. If I wanted to add compression to an original source, say a mic input, is there a method to do so  with these 2 components?

Sure - 
As long as you have open I/O ports on your interface, you can use the External FX plugin that is part of Sonar
(I believe it's part of all the most recent full versions of Sonar).
 
You might have to get creative with the interface I/O routing, but it definitely works.
 
I've used it with a Focusrite 18i8 and an external Lexicon MX300 ~
works great!

Thanks rivers, I think it's the 'creative' part I'm having trouble with. I'll search some more on the internet but so far no luck on the actual routing or actually how to use the External FX feature within Sonar..

It looks like I'm going to bail on the hardware compressor and wait until I actually need one. It served a purpose years ago with my mixer but just not sure it does anymore.