Oddly enough, I have been able to recruit those companions as long as they appear and I recruit them before reloading a save game. After I reload a save game, they're no longer recruitable. Even when recruited, they won't appear in the encyclopedia. What's really cool, is that if you recruit them this way, you can assign all of their stats. For instance, they come at level 11 or so, but all of their state points are not assigned so you can allocate them however you want. To be honest, I prefer being able to assign their stats and wish that was standard and intended. It solves the problem of them having idiotic stats.