Add an Action to ActivityTask?

Hello Guys,

My business requirement is, change the status of account from Activity task ,so I thought it can be achieve if we can add a custom button to "ActivityTask".

but when I am trying to extend ActivityTask to add an action, it is giving me an error as "Keyword 'action' is not supported in non-ESF2 business object extensions"

Can anyone please help me out to get it through?

Thanks in advance,

Nikhil W