Is it possible to add a second domain to an existing Windows 2008 server? This server is already hosting a DC (dc1). I tried
dcpromo and wizard prompted me that existing DC will be erased or inactivated if go through the process.
If you want to have two separate AD domains on a single server, the answer is no - you can't do this. If you want two separate AD domains, you need (at a minimum) two servers.
If you want to add a second DNS zone to a Domain Controller running DNS, you can do this through the DNS manager > Add New Forward Lookup Zone.