Not really... you can be creative tho. You could potentially do this if you replaced the campaigns in the mpq, but even then you'd need to look for more stuff than that probably. I don't think it's worth the effort to be honest!
You'll be better off imitating your objectives somehow, and giving some manual in text files. ("Don't kill the last grunt walled in your base, until you've built a castle on the main enemy island"... or smth)
FYI, there was a program that can enable/disable units in a pud and change projectiles etc... I believe it's called wardraft, but not sure.