A german Thread was created for that. U can use google translate. i hope it helps a bit.
In fact u must modify teh dawnguard.esm to make it possible that u can run it with Enderal. First of all u should try to delete every script and landscape. That comes with Dawnguard.
Ok i remember dont delete every script u need some for some things that coe with armor or weapon. Especially the crossbow
Ps make a save before u do it ^^