Hi,
There is nothing extra needed for configuring PPPOE on the Sub Interfaces.
Only , you need to configure the VLAN number under the Sub Interface and then configure the PPPOE commands.
Refer:-
http://www.cisco.com/c/en/us/td/docs/security/asa/asa72/configuration/guide/conf_gd/intrface.html
http://www.cisco.com/c/en/us/support/docs/security/asa-5500-x-series-next-generation-firewalls/110322-asa-pppoe-00.html
Thanks and Regards,
Vibhor Amrodia