Hello Mike,
here it is the doc
http://www.cisco.com/en/US/products/hw/switches/ps5023/products_configuration_example09186a00807811ad.shtml
To be sure that the new switch will not become the master you should increase the priority of the current master.
see the section
Tips to Add a Switch as a Slave to the Stack
you need to have the right image (the same) on the new switch and you can set to 1 its switch priority.
The other option is adding a provisioned switch to the stack (that perhaps you didn't like)
Hope to help
Giuseppe