If you mean original Lemmix, you'll need the old version of LemSet:
https://www.neolemmix.com/download.php?id=73I don't recall how to use it, but it should have a readme, and you could also just extract an existing graphic set.
If you mean NeoLemmix, there's a graphic set editing app on the NeoLemmix website:
https://www.neolemmix.com/?page=gstoolNote that you don't strictly
need this app; you can do it all by hand with an image editor and a text editor. There should be documentation somewhere, otherwise, study existing graphic sets and work from there.
In terms of getting it added to NeoLemmix, PM it to Nepster. You could also share it by attaching it to a post in the appropriate board (the NeoLemmix Graphic Sets board).