As Parthiban said, Extreme doesn't use sub interfaces. The closest equivalent would be adding the port to the vlan tagged.
The statements "config vlan10 add ports 48 tagged" and "config vlan20 add ports 48 tagged" would cause port 48 to behave the same as a Cisco port using the sub interface command, sending traffic belonging to vlan 10 and 20 on port 48 as tagged frames. The IP address and tag information are defined under the vlan configuration in Extreme, not the interface, as you can see in Parthiban's example above.
