Hi Robert,
There is lots of ways to do it.
If you don't need your 2nd structure to do something with it exept put it in the file. In my point of view I will go for a string table and do a concatenation of all field with separator CL_ABAP_CHAR_UTILITIES=>HORIZONTAL_TAB.
So you won't have to maintain both structures and loop but only the 1st and CONCATENATE.