Diagnosis
Additional fields are always specified in the application design withreference to a field in an existing structure (similar to LIKEin ABAP).
For reasons of stability, reference to fields of generated structures isnot permitted, consequently you cannot include generated objects in thefield selection (hierarchy on left).

Procedure
Every additional field stems originally from a non-generated structure.So you should use the corresponding structure. You can see the origin ofa field in field TCACS_DESFIELDS-FROMTAB ("name of table from whichfield was copied").
If you apparently copy from a generated structure (by vertical drag anddrop in the hierarchy on the right), the "real" original object isdetermined.