A similar topic has allready been made here. Check this link!. I got a nice reply from Thandal in that thread on how to cheat your way around this, but still if anyone would be able to actually make a mod, it would make the game SO much better. Clearly, we are many who would like to see this done! There are so many ways one could work around this, as suggested above a persuade option for Alistair to allow Logain into the party, or as I mentioned in the other thread, conscript Jowan and make him a Gray Warden and have him kill the Arch demon. Or for that matter what about Leliana "I'd do anything to stop the blight", why not make her a Gray Warden? It seems a bit strange that once you actually have the possibility to do the joining-ritual non of your companions seem to even consider it.