There is no need and no possibility to add speakers to the speakers widget. This usually happens automatically.
The only thing what can be configured afaik is, that the widgets can be enabled and disabled.
Devices → Home view → … (top right) → settings → checkmark the speaker widget
If the speaker widget is already marked:
– re-install the Homey App
– get in touch with Athom support
Hmmmm….this morning the speaker widget is suddenly updated with all my SONOS speakers. I haven’t done anything and no new updates (app and otherwise)???