Behind closed doors most of the files and tools from Nextion have been reverse engineered. Some time ago we decided to make most of those efforts public. This includes a tool that allows you to convert TFT files from Nextion to TJC and vice versa!
Some parts are still pretty experimental and not very actively maintained but nonetheless I think this provides lots of value for other people.
Edit: Since the question will probably come anyways: no, we won’t make the tool public that allows converting HMI files. However, the Nextion2Text tool indirectly documents a lot of the file structure and the CRC algorithms involved are the same as in the TFT file (Python code here).