Firstly you would have to obtain the original modelfiles from the author of the Plaxton President for MM2, however a lot of MM2 authors have disappeared, and are unable to be contacted. If you do somehow manage to get the model files, you would then have to export them to .x format, then run them through the converter which is in the OMSI SDK. After that, it is scripting, and configuring the bus, which isn't easy.
However, alot of MM2 models aren't up to scratch with the same quality of the C400R, or my streetdeck, for example. Alot of work would have to be done for it to be a usable state in OMSI, and could possibly take months to complete.