I'm not sure that the SafeMailItem wrapper exposes enough information. Apart from standard metadata such as From, To, Cc, Subject, Date Sent/Received, you will need to fetch back the PR_INTERNET_MESSAGE_ID property (stripping out the angular parentheses that surround it).
I'm uncertain whether or not SafeMailItem allows you to fetch back all that data.
Short of creating your own MAPI object wrapper (a dark art..) you should consider purchasing the
Redemption
library which simplifies access to MAPI properties, in much the same way as the Outlook object model does. You can always use the free version Developer license for your proof of concept.
Perhaps you can even just use the version that is licensed and shipped with the iManage Outlook integration..
I think the SafeMailItem is intended to be used from the scripting facility in iManage WorkSite, i.e. the NRS scripts.
From what I remember, the SendItem.NRS script exposes a MailItem item object, which exposes a SafeMailItem property. This last object allows you to tinker with the mail item properties without raising security warning messages in your Outlook mail client
I don't think it's intended to be used for importing files into WorkSite. As I said, I think you'll need to look at another library, such as Microsoft CDO or (better still) Redemption.
In any case the SafeMailItem doesn't give you all the properties you'll need to properly import an email message into WorkSite. Notably it's missing Date Sent and Date Received values