There is an option to use Hybrid Provider setup and use both LDAP and local users. Though i haven’t tried this myself. And it is kind of clunky (see Speedy reply here Hybrid auth (local + LDAP) is supported).
You can try opening users profile in Admin Console, go to Roster menu and add new item with all@broadcast.server as JID. Maybe it won’t work either because of LDAP. Or maybe you can try adding such user manually in Miranda. At least to try and see if adding such user actually works for sending a broadcast. Can’t help more.