Create deep structure from flat input structure

Hi everyone,

I have a tricky mapping task here, which can probably easily be solved with node functions, but I don't have a clue yet how to get it right...

So here's an overview of the structures:

The <line> element contains a key in the beginning, which determines the type of segment to be created, the rest of the fields within the segment are filled with substrings from the rest of the current line.

As you can see, I managed to map the elements on the uppermost level successfully. I did this using the following function (example for ENT):

The fields within that segment are filled simply via substring:

But the problem starts on the next level. If I try to map the ..._CSG element the same way as the ..._ENT element, there's no node created, like in the data example above. How can I achieve to create sub-elements on the 2nd (3rd, 4th, ...) level with flat source data like this?

Thanks a lot in advance!