Add the film burn clip to a track above your clips and then add the Blend effect from Mixes in the Effects panel to the film burn clip. Now change the method to Add or Screen and it should look like you want. In my test, I keyframed the opacity so that it started and ended at 0%, while in the middle it was eased like a rainbow to 100%. This makes the effect appear more gradually. The default was a little too harsh for me. It's up to you though!