If you want to use the monorail to MK, from BWI, you either have to walk to MGM and take a bus to the front of Epcot, or, walk through Epcot. There is no way to walk to the monorail at the front of Epcot without using your park pass. There should be a bus that runs from MGM (Hollywood Studios) to the main entrance to Epcot though.
Once you are on the monorail line, you will go to the TTC. There you will have to hop off the monorail you are on, walk down the ramp, and go back up the ramp to the MK monorail line...there you have two choices..either the Express which makes no stops inbetween, or the Resort Line which stops at the Polynesian and GF before arriving at MK.
I would use the bus from BW to MK....especially if you don't have park hoppers.