We dont have ability to add custom DHCP options on ePMP devices. You can only enable DHCP Option 82 on ePMP AP.
DHCP Option 82 ePMP inserts “remote-id” (option ID 0×2) to be the SM’s MAC address and the “circuit-id” (ID 0×01) to be the AP’s MAC address. Those two fields are used to identify the remote device and connection from which the DHCP request was received.
On SM in NAT mode you can configure DHCP Local Server to provide IP address for LAN devices. You can found it under Configuratiom>Network>Ethernet Interface>DHCP server.