In short, it's unlikely to get external encoders that will work with the Hickory.

In full, Hickory is Centroid's EtherCAT system, which means technically you should be able to use any EtherCAT drives.
However the choice of EtherCAT drives is quite limited, and most EtherCAT drives are newer designs which make use of absolute/serial output encoders, not conventional quadrature output encoders.

If you could find a suitable external encoder (Leadshine EL7's are using 23-bit encoders), although you could connect it to the servo drive instead of the motor encoder, there may be a couple issues.
First is if the drive is using the encoder to establish the servo motors rotor position, then if you move the encoder to a random position, the motor could run roughly, or just not run at all.
Second is the potential issue that the drive may not handle any additional backlash between the motor and the encoder, leading to performance issues.

If you really wanted to make use of the additional encoders, you'd have to consider something like a Dynomotion Kflop or Kogna, as they can close the loop between the drive and additional encoders.