After reinstalling Symantec Endpoint Protection Manager, the existing installed clients will need to be added into the Manager Console manually. There's no built-in feature/mechanism to search for existing clients and add them from within the console.
To add existing clients, first login to the Management Console. Under Clients, select the group you wish to add the existing client and view the Details tab for that group. There should be a Policy Serial Number field listed under that tab. Note down the first four characters of that serial number.
Next, browse to C:\Program Files\Symantec\Symantec Endpoint Protection Manager\data\outbox\agent. There should be one or more folder with hexadecimal naming in there. Open the folder that starts with the same four characters noted earlier. Copy the Sylink.xml file to somewhere where you can easily access from you existing client computer (pendrive, shared folder, etc).
Go over to the computer that has the existing Endpoint Protection client, and run "smc -stop" without the quotes. The Endpoint client should terminate shortly. Browse to C:\Program Files\Symantec\Symantec Endpoint Protection and override the existing Sylink.xml file with the one you've just copied from the Manager.
Once done, run "smc -start" without the quotes and the Endpoint Client should restart and connect to the Manager.