Add switch to stack

I currently have a stack of 3 3750 switches and I am going to add 2 more to the stack. Because of some spacing and cabling limitations the switches wont be added below the bottom switch now.

It will go from this

1

2

3

to this

1
new switch
2
3
new switch



1) is it possible to add the new switches in without causing downtime to the users connected to the original 3 switches? This will be done after hours regardless i just want an idea what to expect.
2) I assume after they are all up I can just renumber them so the switch number corresponds to its logical position using the switch xx renumber yy command?