One way that I get round adding extra PSTs to X1 is to create a Mail Profile in Outlook, one with no email accounts associated with it, and to add all the PSTs to that. X1 then picks up the profile and its PSTs.
I'm afraid I am not familiar with Exchange and so don't know if my strategy would work in these circumstances.
I'm afraid I am not familiar with Exchange and so don't know if my strategy would work in these circumstances.