Is it possible to block outgoing connections that other Extensions/Plugins are making?
Yes. You will need to look in the behind-the-scene scope though, this is where these networks requests appear (use the logger to see them).
However, before you can block behind-the-scene network requests, you need to enable advanced user mode: blocking indiscriminately behind-the-scene network requests – which also comprise those made by the browser itself – can “break” your browser, hence this is reserved for advanced user, and whoever filters behind-the-scene network requests is on his own to fix issues arising from this.
Also, the behind-the-scene scope is whitelisted by default, so it would have to be removed from the whitelist pane.
See https://github.com/gorhill/uBlock/wiki/Behind-the-scene-network-requests.