How to import Distribution Lists from Outlook into Windows Mail?

When I exported addresses from Outlook and imported into Windows Mail, the Distribution Lists/Contact Groups disappeared. I have the lists in tab-separated format – how can I use that rtf or txt file to set up a Contact Group in one go in Windows Mail?

Come to think of it, the Distribution List only shows up in Outlook when I start typing its name in the To: field – it’s not actually there when I open the address book.