It is possible if you export users emails via admin panel and then use some kind of a special mass-sending program which are quite a lot on the market (as it was discussed previously
If you mean is miniBB providing such feature by default, the answer is no
First of all, sending mass-emails most commonly means a spam, and we are not a spam software. Sending something to all users at once is only possible using internal Private Messaging system
(there is a special function for admin).
Second, sending emails through a PHP script is not effective nowadays, since such emails in many cases will be blocked by spam/flood filters on the user's server's side, and often they even do not reach the recipient.
Third, it is often disallowed on the hosting server, by the hosting rules, that you can send massive emails that way. Actually it is a law violation in many countries, so we won't support such feature, never.