How to add a third-party repo. and key in Debian?

I looked at another similar question about adding third-party repos. I am trying to add a third-party desktop IM client called riot . While the site gives link to the third-party it gives no instructions as how to add third-party sources or keyring in Debian. I went through https://riot.im/packages/debian/pool/main/ and made the following additions in my /etc/apt/sources.list –

Setup Debian Mirrors after Installation

Suppose I install debian,and my Internet network
comes down.
Install works OK,but at the time to setup the apt mirror
from list give error because network Internet is down.
I continue to install without mirror(apt/sources.list contain
only cdrom entry)
Internet work..how to setup the debian mirror after installation?
I know how to edit sources.list with vi,but I want the menu with
mirror list selection.