question about structure and additional field declaration

I want to have a structure with a few additional fields so

the code below works

data: begin of i_catsdb occurs 0,
         vernam type ernam,
         versda type ersda.
         include structure catsdb.
data: end of i_catsdb.

but why can't I declare the fields after the structure ?? like below or some other way. I couldn't find any working solution with , en . on different places ?

data: begin of i_catsdb occurs 0,
         include structure catsdb.
         vernam type ernam,
         versda type ersda.
data: end of i_catsdb.

is there a reason behind it or is there some other way to append the fields after the structure ??

kind regards

arthur