Bluesound are fully RoonReady, so they are unequivocally RAAT Zones.
The KEF LS50W, however, are a bespoke integration meaning they are not Roon Ready and do not use RAAT in the usual way.
The Build 269 release notes said:
Also, note that due to the technical details of KEF streaming these speakers are not Roon Ready, so linking multiple pairs of LS50w is not possible and some RAAT-specific functionality (like convenience switching or advanced Signal Path functionality) is not possible.
I suspect that being unable to link multiple pairs would also preclude linking to other RAAT Zones but I haven’t any personal experience. Let’s see if @support can say for sure.
One option is to use a RAAT endpoint to feed the KEFs (or the Mojo) instead of direct Ethernet connection. That would be capable of being synced with the Bluesounds.
The choice of endpoint depends on what connection and resolution you need. At the lower end there are Raspberry Pi based devices that perform very well. I use an Allo DigiOne in one system and find the sound good up to 192 kHz, but it falls over every now and then if left on. I think it is a known grounding/static issue that can be resolved by feeding it a separate power source.
In my main system I use a Sonore microRendu which does Ethernet to USB very nicely up to 768 kHz or DSD 512.