Yes u can di all this with CGI.
the first thing is to choose the language for cratin your CGI, i think Shell (Bourn or Korn ) will run well.
for creatin for example user just save in text file
user.cgi :
mkuser (AIX), useradd or adduser syntax
eg. adduser -u login -......
Don't forget to change mode : chmod o+r user.cgi
and thant just save it under :
.../cgi-bin/
your link will be :
http://...../cgi-bin/user.cgi
good luck