Add eero to Airport Extremes

Not directly with the Airports.. you will need to run ethernet to the main Eero and then can set the others to mesh with it.

Really difficult to mix and match .. mesh is not at all like old Apple extend wireless.. so I think you are better keeping them separate .. or totally / partially replacing the Airports.

Not necessarily recommending that you do this, but if you only want to add another AirPort........refurbished and used AirPort Extremes are available on eBay.

On the other hand, if your current AirPorts are all 5+ years old, then it would probably make sense to replace them with newer products.