Hi,
Generally speaking, configuration manager is application level and it's authentication mechanism relies on Active Directory mostly. If there is a 2-way trusts between the 2 forests, it would be better and we can place a DP in the remote forest more easily.
For this problem, the configuration manager expert Jason has written a great blog and we may find both the short answer and detailed steps.
If there is no trust, we can add the target domain to the exist site and install the clients manually to manage them. Here's a step-by-step guide.
If bandwidth is a concern, we may also consider to just retrieve policy from the SUP and get the updates from Internet.
Alex
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