It depends on the contents of the hex file and what you want to achieve as M-file. Actually .hex files contain binary values in hex format. e.g. "4A 61 6E 20 53 69 6D 6F 6E" or the same without spaced. But this cannot be converted to an M-file directly. The hex code is only useful to transport binary data through a connection which allow 7 bits only.
Perhaps your hex files have a more complicated inner structure, e.g. https://en.wikipedia.org/wiki/Intel_HEX . Then please explain what you mean by ".hex file" and what you expect after a conversion to an M-file. Maybe reading the data by fscanf(fid, '%2x') is sufficient already to import the data as uint8 values.
