Recover private key for a webserver certificate that was issued to a server in a different forest?

Environment: Sub/Issuing CA is Windows Server 2008 Ent R2 in Forest A

Few years ago, A web certificate was requested from forest B using enrollment web site https://web.foresta.org/certsrv. The cert was issued and is being used since then.

How can i make sure the private key for the web cert is stored in the Key Archival database on my issuing CA? If not, how can i archive the private key so in case something happens to the server, the private key can be used to recover the data.

thanks,

Navgup