How do I do that? I use BFGMiner.
I've never used BFGMiner, but there are probably similar options available. Here is how I would do it with cgminer:
Run this command in the cgminer directory: cgminer -n
That will list all of the available devices. Edit your normal start up command line and add -d #,#,# to specify each video card you want to mine with. Replace the # with what ever number cgminer used when it listed your cards. An example might be -d 0,1 or -d 1,2,3. Start that instance mining just like you normally would without any BFL hardware.
Create a new worker on Slush's pool and make a new command line for that worker, but add -G. -G tells cgminer to ignore any video cards and only mine with other available hardware.
Again, I've never used BFGMiner, but I assume they operate in a relatively similar manner. You might need to read through the documentation if it uses different command line arguments than cgminer.
Good luck!
Chad