I just figured out how to play a custom CTF level in JJ2 Shareware. You can have only 1 level though. All you have to do is download a CTF level. Then make a back up of Sharectf.j2l which is in the JJ2 Shareware folder. Then copy the custom level to your Shareware folder along with its tileset and music. Then rename the level to Sharectf.j2l and run JJ2 Shareware. And you can run the custom ctf level in the JJ2 Shareware. Do the same for treasure except the file is called Sharetrs.j2l

I tried it with Single Player but sadly, only certain levels will run. For example I tried Tubelectric and I got this message "The level you tried to access contains a refrence to the animation 'PINBALL'
which is not in the shareware version of Jazz Jackrabbit 2.
You are not able to play this level." So far we only know that levels with pinballs in them won't run. To do it for Single Player you do the same thing except the files are called Share1.j2l, Share2.j2l and Share3.j2l. The downside is that you can only have 3 SP levels.
I hope you liked this thread and Happy Jazzing!!!