Since the existing stack and the new one may have different firmware revisions, typically the best route would be to update firmware on the existing stack and then on the new switch to be added before attaching it to the stack. You should not need to do any initial configuring before attaching. As always best practice is to use a ring topology when stacking to avoid split-brain and duplicate IP address issues that are common with daisy-chain failures.   -B     
Hello I have a new N3048 switch to add into our current stack of 2 N3048's to provide more capacity. Before switching on for the first time I would like to know if I need to run through the initial config or can I just connect the new switch to the stack and power on, will it then get the config from the master switch?  The firmware also needs updating on the stack, should I do this first or when all 3 switches are in the stack?  Many thanks. 
Add new N3048 to current switch stack