As already stated, the CP441-1 does not support the standard off-the-shelf loadable Modbus drivers, you will need the CP441-2 for that, and also the license to use the driver that takes the form of a plug-in dongle.

You, as you suggested, used the free port mode for the CP441-1 and write your own Modbus protocol driver using program blocks and the standard CP441-1 interface functions (BSEND / BRECV).
However, be aware that this will take quite some effort and time to make a good reliable solution. I know this as I did my own some years back. Plus, you then have the task of validating the solution and gaining confidence with it. It doesn't end there either as you will then have to support it, as it is your code. You will get limited support from the official Siemens channels for your own solution.

Whichever way you got, I wish you success.

Smiffy.