New generation type with custom map

Hi, I have read about this in the forums before but I struggle a lot. I want to generate a dimension with a personalized map. From what I understood in MCreator this is impossible, so the solution I experimented with is to create an empty dim and generate a
structure (32x32x32). The latter generates with the correct coordinates of other structures and etc. However it works well on a small structure but for a card it is not an option.
Hence my question: does anyone have any ideas to change the generation of dimensions? (I have good basics in java so no coding problem) Or another idea. Thanks in advance for your answers