NEC 4 Option A - ECI?

We are tendering a project where the ITT refers to the main contract being awarded as an NEC 4 Option A. In addition, the ITT then mentions the option of an ECI for the successful tenderer. NEC4 only includes X22 in Options C & E. What would the best method be to include an ECI phase with an Option A contract ? Could it be done by a stand alone Professional Services Contract instead ? Would it need to be a separate form of contract ?