You're far and away better off getting an old scanner and doing that - why void the warranty so soon (unless you've had that for awhile, and don't care that you would void the warranty if you did this)?
Maybe this will work for you. I use a PSR-400 with UniTrunker. I connect the ext. speaker output to my computer and turn all the volume up and all the squelch down. I get a good signal for the software to start decoding.
Hope this can help you.