Have you found a format that both programs can work with? I don't know of one for Mailing Lists.

The best you can probably do is to export them to a CSV file and then see what you can do with it in Outlook.

I was able to export/import contacts EXCEPT for the distribution groups.....they did not show up in csv file.

Mailing List and its equivalents in other email clients are not interchangeable; there is no standard format to express them. The variety of names used sort of hints at this: Groups, Distribution Lists, Mailing Lists. All designed for slightly different purposes and all implemented differently.

The nearest we have to a common format for data exchange is the CSV file, which most address books can make sense of. But the CSV file is a "flat" data format and cannot (easily) represent nested data.

And note that in Thunderbird's Address Book, a Mailing list is only a subset of the parent address book. It is not in itself a complete set of data about Contacts, just pointers to the corresponding entries in the host address book. So at one level, a Mailing List is pretty useless, but a rational exporting process would and should use it to garner information from the Contact a Mailing List entry points at.

The MoreFunctionsForAddressBook add-on provides an export tool for Mailing Lists, but since it uses a fairly unusual file format (.thm), it is only really useful to transfer your Mailing List data to another installation of Thunderbird.

If whatever system you're using permits you to convert Mailing Lists into address books, you'd find all this importing and exporting slightly easier.

But in a quick test here, if I select a Mailing List and then go to Tools|Export, I get the option to export the data to a CSV file. What more could you want?

When you import address data into Thunderbird, it always creates a new address book. You then need to drag-and-drop its contents to wherever they are useful to you, including a new Mailing List if that is appropriate. Note that adding new Contacts directly to a Mailing List in Thunderbird will automatically generate entries in the corresponding address book.