AWS Batch use existing ECS cluster
I don't think what you want to do is possible. 

I suggest you to use a managed compute environment so you don't care about cluster resources. Set the min and max CPU for the compute environment (min 0 is recommended so you don't waste many when no jobs are running). Then only set the resources needed by your jobs and batch will add compute capacity to satisfy the demand. 

Your costs will be the same because batch will create an ASG(autoscaling group) and ECS cluster but it only charges you for EC2 instances.