To stay in Blockly for something unsupported like that you can use the inline script block and enter any valid ECMAScript code.
It’s also possible to create a Block Library (Developer Tools → Block Libraries) where you can create your own Blocks that cna be used in your rules. Though that’s a bit more advanced of a feature and requires a good bit on knowledge of how Blockly works. There are a couple useful tutorials on the library and you could use many of the libraries posted to the marketplace.
Speaking of the marketplace, there actuall is a Block library for XMPP on the marketplace already. XMPP Actions
Theoretically, you can install it from Add-on Store → Automation → Block Libraries and once installed you will find it under “Libraries”. But this library doesn’t have the “published” tag for some reason. If you navigate to Settings → Community Marketplace → Show unpublished entries that should show the XMPP Actions entry in the list.
Once added you’ll see a “Libraries” option on the Blockly screen and you can access the blocks from there (I only have the OHRT Block library installed).
I don’t have xmpp so I can’t test it out myself. But if you test it and let me know if it works I can add the puiblished tag so it’s more widely available.