Thank you for using wpForo.
User passwords will be preserved if you migration from WordPress plugins like bbPress, Simple:Press, Asgaros or from any Q&A forum plugins because user passwords hashing/encrypting algorithm stays the same with WordPress functions. However, if you migrate from stand-alone forum software like phpBB, SMF or MyBB there is no way to preserve passwords. The password hashing/encrypting algorithms are totally different and they don’t match to WordPress. There is no way to decrypt password hashes, this is one-way encryption process, thus all users password will be changed. Users should reset their passwords using “Lost your password?” link located under the forum login form. You also can add a small note/message to forum users and put the reset password link in that message on top of your forum home page using Admin Note tool in Dashboard > Forums > Tools > Misc admin page.
The information is already mentioned in doc:
In case you want to say thank you !)
We'd really appreciate and be thankful if you leave a good review on plugin page. This is the best way to say thank you to this project and support team.