If you created the domain libertypark.com with an info user then create a user alias for libertypark on the email.com and set it to point to firstname.lastname@example.org, it would work. Make sure that you copy the email@example.com user data files over to firstname.lastname@example.org before removing the email@example.com user.
1. Create libertypark.com domain;
2. Create a info folder in the libertypark.com\users folder;
3. Copy firstname.lastname@example.org data files over to this newly created folder;
4. Open the userConfig.xml and search for libertypark and replace with info for all applicable occurrences (username, contact info, popup alerts, etc.);
5. Goto System Admin -> Manage -> Restore, select Attach user and enter email@example.com
6. Press Restore
7. Remove the user firstname.lastname@example.org
8. Create a user alias libertypark that points to email@example.com
Any messages sent to firstname.lastname@example.org will be directed to email@example.com.
I hope this helps.