Adding new items in existing uicomponent?

Hi.

I am beginner on SAP.

I try to add a new Action button on another BO of existing QA form in UI Designer.But I switch to Edit mode, error message box "Component which you are trying to edit comes from a lower layer.Please use Extensibility Explorer to edit." is shown.I can't understand this is why and how can I add new tools on existing QA,OWL or OIF?

Thanks.