How to make a bip file including another 3 bip files in KEYSHOT

i have 3 bip files that i have already finished all the materials and color stuff. Now i want to get together them. If i put them in an assembly on Solidworks and save it, after that I 'm gonna suppose to reapply all the materials and layers again. I've tried this operation by using import but i can't do it. Which option that i have to select, I dont know. Help me please, Best Regards...