How do I add sites associated to a different hub to a hub's search scope in SharePoint Online?

Hello Thomas_Randall 

i think it's not possible, the Microsoft Search is scoped only to the Hub, but you can't add elements from other. But you can user the PnP Modern Search WebParts to show more elemnts from different sites.

https://microsoft-search.github.io/pnp-modern-search/

Best, Dave