Implicit Enhancement of an internal table

Hi,

I've been trying to add a field to an internal table via an implicit enchancement but cannot get it to work - not sure if it's even possible:

data: begin of i_tab occurs 0.

     include structure mara.

data: field1,

        field2,

enhancement 1 zz_itab.

        zfield,

endhancement.

end of i_tab.

The error is: "Comma without preceding colon (after zfield)"

Thks,

William