Here is the reply I received from WaltLab.

Hello Bill,

Thank you for your inquiry and interest in our products. A direct data
import from UBB.threads is unfortunately not possible. The Forum must be
converted into a phpBB at first. Only then the data can be transferred
in Burning Board.

A converter from UBB.threads to phpBB 3.0 can be found here:

We would be happy take on this work for you. In order to provide you an
individual offer, we need the information how many users, posts and
attachments (total memory) need to be converted.

If you need any further assistance, please do not hesitate to contact us.

Yours sincerely,

Andrea Berg
Customer Support WoltLab GmbH