CC2538: network join rssi / lqi threshold setting

Hi to All, 

I use Z-Stack Energy on IAR, CC2538.

Our network is set to auto-PAN, and we control which router / end-device joins which coordinator by enabling the joining on the coordinator with NLME_PermitJoiningRequest(  ). But we have had issues where this is left permanently enabled on some coordinators due to user negligence, and so this causes unwanted joining.

Is there a way to base network joining on the distance to the coordinator too? (rssi / lqi threshold)? If not, which function could I edit to build in this feature?

Thanks for your time!