I think this is what you want:
adduser --disabled-password --shell /bin/bash --gecos "User" $username
You'll be able to
su $username
to run commands as that user, in /bin/bash/bin/bash or whatever shell you specify. --gecos "User"--gecos "User" just put "User""User" into the comment field so you aren't prompted for that information. You can put whatever you want into that field.