I couldn't get a different stage to work either, so it seems to be 1 of 2 things that could be wrong.
1) my rel file isn't set up correctly
or
2) I am missing some cheat to make the extra stages playable, much like Giga Bowser, Alloys, etc.
I don't think you can use Bero's tool to make a st_custom#.rel. I have purl=http://frhed.sourceforge.net/]Frhed[/url] installed to do this.
You are correct, it doesn't. I used Ultraedit to hex edit them using the module files info on the page posted by Chuy. I do have some experience with hex editing, but I could still be doing something wrong. You see, I tried 3 different hex editors that I have, & only 1, Ultraedit, would find that string. I'll also try the hex editor libertyernie posted.
EDIT: I finally got it. Frhed didn't work, because it seemed it wouldn't open the full file, so I used Ultraedit again. I apparently accidentally changed the wrong value the first time. Thanks guys!
Also, again, is there any thread that is collecting the rel files for others, that I could donate to?