What software do you have on your system, and what release of Media 100? One way would be to combine the Fill and Key in e.g. AfterEffects into a single movie in Media 100 i codec with alpha channel, which then can be placed on the Graphics track. You also can do that in e.g. Boris Red if you have it. You also ca do that in the timeline in Media 100 i, but it is not that easy: Place your background on Va, your Fill on Vb and create a Transition between them. Use BorisFX for your transition, place the Vb over Va inside the transition, and use the key file via the Add Track->QuickTime import in BorisFX as a key (with a linear Luma Key).