Look for a option to do picture-in-picture in your video editing software.
Most major software allows you to click and drag the size of a video. So, you grab one of your previous videos, add it to the timeline just like any other clip. Put it at the end of the video and resize to match your outro open space. Then stretch it on the timeline so it's visible when the outro is.
Thanks for the help guys. I managed to figure it out.Use a multi track software (I use vegas) then you adjust the video track's dimensions to the outro's dimensions (Not sure of other softwares but vegas does this video track tip )