Not sure about the beckon ability, but you could try copying the corruption from Belial :
http://dom3.servegame.com/wiki/Lord_of_Corruption
It works like seduce but can works on any unit (I think).
Problem is the ability is not called "seduce" anymore, which would have been appropriate for desire, and I don't think that there is a #corruption mod command (unless it is what #beckon do ?) and doing a copystats from belial might come with other undesired flags.