Add Column in the Powl list

Hi Experts ,

   I need to add some custom column in the POWL list and for that I already did the following steps.

I am getting error while try to call get actions method for calling the custom POWL application.

Steps -

1> Created custom structure and include  USMD_S_CREQUEST_POWL structure

2> Created custom TABLE TYPE and maintain custom structure as LINE TYPE

3> Copied  feeder Class CL_USMD_CREQUEST_POWL into custom feeder class.

4> Maintained custom application in transaction FPB_MAINTAIN_HIER

5> Created powl type in transaction POWL_TYPE with custom feeder class.

6> Created powl query in transaction POWL_QUERY with custom query id & powl id

7> Created custom COMPONENT CONFIGURATION from WD component IBO_WDC_INBOX

8> Created custom APPLICATION CONFIGURATION from WD component IBO_WDC_INBOX and assigned configuration.

9> Maintained application , role , powl type in transaction POWL_TYPER

10 > Maintained application , query in transaction POWL_QUERYR and activated.

11 > Maintained Application configuration in role .

12 > Added Custom coding in method GET_OBJECTS & GET_ACTIONS.

Got error  ' Object does not contain an interface '  in the follwoing method calling -

CALL METHOD super->if_powl_feeder~get_actions

EXPORTING

i_username = i_username

i_applid = lv_applid

i_type = lv_type

i_selcrit_para = i_selcrit_para

* i_langu = SY-LANGU

* IMPORTING

* e_actions_changed =

CHANGING

c_action_defs = c_action_defs

Please guide me to resolve the same or the way I can add the custom column in powl list.

Thanks in advance .

Samrat