Unable to exclude accounts from Forest B from the local ChangeAuditor client installed in Forest A.
ChangeAuditor queries Active Directory for local Domain users when selecting accounts for exclusions and by design users from other forests are not available for addition.
Install a ChangeAuditor client in every forest where an account resides that needs to be excluded. Exclude the account from the newly installed client in the remote forest. This will allow the client to see the users in that domain and add the exclusion template which will be seen from the local client as well.