One approach would be -
“Make your own Modbus gateway”
You would need a rpi script that talks serial Modbus to your meter, as a Master. And talks Modbus-TCP over ethernet, as a Slave, which can be polled by openHAB.
Ready made gateways are relatively cheap if you do not have the skills.
Alternative, a Gateway of any other flavour you like. A script of yours talks Modbus to the meter and transmits the data using your choice of protocol - MQTT perhaps.
The over-engineered version of that is to install a complete openHAB service on the rpi to read the meter, and use the Remote binding tolink to your main openHAB instance.