about useradd accountname rules

How to let useradd accept the login id which include the dot "." ???

I have a box Redhat 7.3 which has "shadow-utils-20000902-9.7.i386"
There are many user id has "dot", such as "aa.bb.cc <http://aa.bb.cc>" and 
everything works fine.
 Lately I install a new box using Centos 3.3 which has "
shadow-utils-4.0.3-23.08" and the useradd don't accept the dot in userid .
 Do you have any suggestion on how to tranfer the user from old server to 
the new one?

Can I use the old "useradd" file at the new box????