Connect pushbuttons

At this moment, I run openhabian 3.2 next to my pc with my old system.
I want to move all to openhab. But i don’t know how I can add the house pushbuttons. In each room the 4 or 5 switches, to expensive to replace them.
These are pushbuttons with a resistance. They are connected (10 lines) to a older microprocessor (basic32). He reads them after an event has occurred with a capacitor and trough rs232 the pc is informed.
I know this will give some work, but better a good solution.
Can I connect this Rs232 to openhabian?
Or better (harder) to create a custom mqtt client?
Any thought’s? How are yours classic light switches connected?

Kind regards
Pieter