If you are going to fill that much, the soil that you are importing will kill off the turf under it anywhere that the depth is more than an inch or so, the exception being if you have quack grass or something similar. Roundup is a non-selective herbicide that will have effect on most (not all - see product label to find out what it is registered to control) weeds and grass types. You could do a blanket application to kill everything off and then, after seven to ten days, re-seed or sod.   _{Your Comment}