In Keynote 9.09, within my presentation, how do I create transparent slides to show a screen feed which is layered underneath?

In Keynote 9.09, I'm developing a looping slide presentation for use in the lobby of a client's business. It will need to be layered over an ongoing screen feed service that supplies graphic images of updated weather, stocks and news info. I need to create a gap…about 15 seconds…of transparency so that the screen feed will be seen. I then need to export the Keynote document into Quicktime so it will operate as an MP4, which is compatible with the software of the digital screen service the client is contracted with.

First - how do I create 15 seconds of transparency with Keynote?

Second - Will the transparency transfer to Quicktime? If not, how do I create the transparency in iMovie?

Thanks.