[sarcastic mode] nope no possible way, the signs are hardcorded into omsi as well as everything else, nothing is customizable! [/sarcastic mode]
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You have to edit the HOF file in the respective vehicle which you want to edit folder. The easiest way to edit the signs would be on a non-roller sign bus.
The first thing you do is go to the Omsi\Vehicles\MAN_SD202 directory, look for the HOF file, if you want to edit the HOF for let say, the Grundorf route. Then edit that file (Grundorf.hof). It is important that you follow the pre-outlined format of the file. To start off just make edits to the display