I think the main problem is that in a subcircuit, the connecting lines and layout of an equivalent schematic are basically missing.
So only way I know of is to go through the subcircuit line by line, and laboriously create the schematic drawing of that in LTspice schematic editor 'by hand'. The nodes (connecting lines) will have numbers in the subckt which can also be named the same on the drawing which does help.
Unless anyone knows differently :D