Arcserve Client Agent for NT

I have one NT Server with Arcserve 6.61. The IP address of the server is 10.40.x.x. The Client Agents work perfectly on the other servers in that subnet within the same NT-domain. I have also a few more servers with IP address 10.41.x.x which do notbelong to the domain but to a workgroup.
I can ping them from the Arcserve-server, but Arcserve does not recognize the Client Agents on these servers.
How can I accomplish to backup these servers without a second tape drive?