Automatically converting a text-representation to a machine-readable representation is harder than it looks. It would require some careful parsing.
Since you're hard-coding the MAC address into your sketch though, you don't need to worry about that. You can simply write it as an array of hexadecimal literals:
uint8_t mac[] = {0x00, 0x1E, 0xC0, 0x04, 0x9F, 0xF3};
In C++, an array type will happily decay to a pointer, so you should be able to pass it directly to your function like this:
network_set_MAC(mac);