Manage another user's credentials for network access

If you can access the remote computer, you could add your service account to the local Users group and match the username/password to what you would use. Don't forget to give it administrative privileges.

Then goto the remote login tab in the system menu, and add that user as someone who is allowed to remotely login. This is what I do when I need to hit certain machines that are on our network, but not part of the domain for whatever reason.

Although it turned out to be irrelevant to your situation, for the benefit of future searchers, the answer to the actual question "Manage another user's credentials for network access" is:

runas /user:serviceaccountname "%windir%\system32\cmdkey.exe /add:server.domain.com /user:username /pass:password"

This will create a credential in serviceaccountname's credential vault for server.domain.com using username/password.

/user also supports domain\username and username@domain styles.