FFMPEG: Add outro to the end of video
You want to concatenate the videos. see
https://www.ffmpeg.org/faq.html#How-can-I-concatenate-video-files_003f

I don't know what OS or multimedia containers you have, so read that FAQ. Especially if you want to avoid making potentially huge intermediate files.

In Linux/Mac OSX you want something like:

ffmpeg -i input1.avi -qscale:v 1 intermediate1.mpg
ffmpeg -i input2.avi -qscale:v 1 intermediate2.mpg
cat intermediate1.mpg intermediate2.mpg &gt; intermediate_all.mpg
ffmpeg -i intermediate_all.mpg -qscale:v 2 output.avi


On  Windows I think the syntax is:

ffmpeg -i input1.avi -qscale:v 1 intermediate1.mpg
ffmpeg -i input2.avi -qscale:v 1 intermediate2.mpg
copy intermediate1.mpg+intermediate2.mpg intermediate_all.mpg
ffmpeg -i intermediate_all.mpg -qscale:v 2 output.avi


If your multimedia containers are already MPEG-1, MPEG-2 PS, or DV you can skip the transcoding steps and just concat using cat or copy or better yet concat protocol.

You can get fancy with scripting to add pseudo-randomisation to add the Outro. But I'd just add outroA to all odd numbered files and outroB to all even numbered files.

If you want to add more that one outro to your content randomly, I'm not that good a command line hacker to pipe and nest randomisations. So I'd build the randomisation (aka spinning) of 100 cat/copy/concat lines into a script via a spreadsheet, then copy/paste the script into a batch/shell script.