Hi Vijay,
You have several options, is this exchange 5.5 server staying in production, if so then just create contacts in the Exchange 2003 org/Forest for each DL in Exchange 5.5.
If this is a one off migration and you will be shutting down the Exchange 5.5 server then take a look at the following procedure from fellow Exchange MVP Rui Silva http://msmvps.com/blogs/ehlo/archive/2005/04/21/43813.aspx
Just make sure your users are migrated and have the same usernames.
Regards
Ben Hoffman
Microsoft MVP - Windows Server System Exchange