Yes you are correct in that you need a certain ROM in order to even use WiFi calling, but if you do have one of those ROMs then you can use the MyMetro app to update/change the E911 address. If you have T-Mobile then you can't use the MyMetro app to change the address, likewise if you have MetroPCS you can't use the T-Mobile app to update the service. And as you stated, you could just call 611 to update/change the address.
Now judging by the OP's question I am assuming they have the proper ROM for WiFi calling and are just having difficulty setting it up and using it because they can't set the E911 address. One final note, it doesn't have to be a stock ROM. I'm running Wicked ROM on my s4 and it has WI-FI calling support as well as WI-FI MMS support. So do look around, but Stock is probably your best bet.