Convert file *.asm in file *.obj

Hi! Escuse me for my bad english. I must convert a model (*.asm) build with Pro/E in a file *.obj. When i convert this assembly in *.obj the tree of assembly not conserved. I view all the part as single part, I view the in sub-assembly created in Pro/E as single part after the conversion. i ask as i can maintain the tree when i convert the file. Thanks