You use a different program other than WadAuthor, like XWE or DeePsea. In DeePsea, you go to the "compile files into an iwad" option, and I'm not really sure about XWE. I think theres a button there that adds a new file one by one. Which is lame.
EDITED POST TO NOT MAKE A DOUBLE POST:
Can you make new "things" for SRB2? Like other than editing a crawla, can you make a new type of crawla? And if you can, how?
EDITED POST TO NOT MAKE A DOUBLE POST:
Can you make new "things" for SRB2? Like other than editing a crawla, can you make a new type of crawla? And if you can, how?