I’d like to use PureOS repos in Ubuntu, how would I add the new alternate ones stored at the sonic ISP repo… Or even just the regular pureOS one…
I am not convinced that this is a good idea.
If you want to try out PureOS, you can probably install it alongside Ubuntu (assuming you have sufficient disk space) i.e. dual boot - either on different disks (safer) or on the same disk, depending on what is available to you.
An alternative could be to use a VM (and run one inside the other).
I wanted to take a look at the Librem apps (like Social)
This is probably a dumb way to go about this…
From https://puri.sm/posts/introducing-librem-social/ it is possible that your choices are to use the web interface (on just about any computer) or specific apps for iOS and Android.
Anyway, to add the repositories you should do it in
/etc/apt/sources.list and add the key but you’ll need to add some apt pinning rules to avoid break everything.