How to get Squirrelmail
 

I did ISPManager install and setup the whole site. But now I realized I am missing Squirrelmail in my install. I need this tool for webmail.

How do I add Squirrelmail to my ISP Manager install?

If I do re-install of ISP Manager, what happens to the site configuration such as WWW and Mail Domains, Users etc...

Whan operation system you use?
Try to install Squirrelmail from ISPmanager - Server configuration - Features menu.
Also problem may be exist if your system have not Squirrelmail package in the system repository, if this true you need make installation manually by himself.