Convert Orbi 870 router to a satellite?

No. 

A 2nd could be used in AP mode. 

Not sure if a 2nd could be used as an AP. When I added different BE orbi routers (one in router mode, one in ap mode) it created significant instabilities and issues. 

Hello Annoosse​ ,

Welcome to the NETGEAR Community!

I understand that you would like to extend your network coverage range by adding another router to your network. Though using the second router in AP mode can extend your network it will not work as a whole mesh system solution. Establishing a seamless network using two routers will take advanced network configuration. It is recommended to use a satellite that is compatible with your RBE870 which in this case is the RBE870 satellite.

Best,

Kevin

Community Team 

KevinLiT​ Does it actually work? I've got several BE systems and whenever I add a 2nd in AP mode, it creates conflicts with the satellites and instabilities. Has this been resolved? 

Kevin did not recommend a second router in AP mode.  He recommended another satellite, and got tripped up by Netgear's goofy decision to number the router 871 and the satellite 870.

i.e.

Every Orbi system prior to BE:

RBRxx0 - 'router'

RBSxx0 - 'satellite'

RBKxx2/3/4 - 'kit'

BE systems, x71 - router.  x70 - satellite