Can I spray urethane foam insulation in cavities that already have insulation in them?

I have a 100 year old farmhouse that I love, but it is in need of some efficiency upgrades. I live in WI, so winters can be bitter cold. This year we had a three week stretch with sustained temps in negative double digits, which is hell on my energy bill, not to mention resource usage! Some of the exterior walls have had cellulose very loosely blown into them, I'm sure with voids. I can feel cold air infiltrating the walls on a cold day... and we're talking actual drafts, not just radiant (I am a professional contractor, and I know the difference! ;) ). Some of the walls have been torn apart, insulated w/ R-11 fibergl, and remodeled. I'd like to insulate with urethane to tighten up the building envelope and increase my R-value before I start getting into more efficient heating systems (low hanging fruit first, right? - Windows and doors have already been taken care of). Can I use slow rise urethane in a cavity that already has insulation in it?