This article says the hair loss is only temporary and that when you STOP taking it then your hair will grow back. So from what i gather, it will slow down or stop after a while (Still taking Imuran) but no new hair will grow back until you stop taking it.
http://my.clevelandclinic.org/transplant/services/lung/azathioprine.aspx
http://my.clevelandclinic.org/transplant/services/lung/azathioprine.aspx