Compress emails in XML format to MBOX

We are migrating away from an old collaboration system called OpenText FirstClass. It's a beast. I need to migrate our users' (many) emails out of FirstClass and into Office 365 (Exchange Online).

The issue is getting the email files out of FirstClass. There is no useful export option, just a convoluted format where each message comes in a folder with the content in XML, RTF, and plain text format along with any attachments. Is there a way that I can compress these XML files into an MBOX or PST format that is actually useful for import into Office 365 or ANY other email system? Or am I just stuck with this odd email-folder structure and no real way to do anything with it?

Thanks!

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