The only way to utilize a wildcard certificate on a SQL Server 2012 installation is to manually edit the registry.

Open up the follow key in regedit: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL11.MSSQLSERVER\MSSQLServer\SuperSocketNetLib.
* Note, if you are setting the certificate for a named instance of SQL Server, adjust the yellow portion of registry path, listed above, to match your SQL Server Instance Name.

Look for the "Certificate" value within this key. If it does not exist, right click the right pane, select New -> String. Set the name to "Certificate". Select "Certificate", right click and choose Modify. Paste in the SHA1 hash for your wildcard certificate, with no colons, no spaces.

Now, look for value named "ForceEncryption". If it doesn't exist, create it as a new DWORD. Set the value to "1".

Now, restart your SQL Server instance and you should be good to go.