I did this recently with an MS410. I had I one running as the distro and I added another one to it. It was interesting, while Meraki support said I will lose the config when I stack them that didn't exactly happen. Here's what I did and what happened:
1-Bring the new switch up have it check it with dashboard
2-Make sure both switches are on the same version
3-Power both switches OFF and cable the stack
4-Power the switches back on
5-Go to switch ---- switch stacks and provision the new potential stack
6-Dashboard will prompt you if you want to clone or ignore the configuration to the stack. Obviously you want to clone it so you don't lose it and I cloned it.
The Stack came up just fine but here's the issues I ran into:
1-The sapnning-tree priority went back to default so all my access layer switches had spanning-tree related issues. What I recommend is shutting down the downlinks to you access layer switches, once the stack comes back up lower the priority on it again and turn your ports back up (that's if you're stack is a distribution ofcourse)
2-I lost all my aggregate ports configuration that I had on switch 1, I had to rebuild them
3-And last when you stack them the L3 vlans will have a new mac-address so flush your ARP table and such.
Thanks