This is a good question that has not been asked in the forum. At the moment there is no autmoated way of migrating from TELSA to ST-Sim but is has been done by researchers at NCSU. There are two possible approaches that you could use:
If you have knowledge of the TELSA database and SQL then you can design queries that export the data to a suitable format that can be imported to ST-Sim using the excel import templates for each model property.
In either case you will also need to create raster based spatial model inputs rather than vector based ones.
I would be happy to help with specifics if you or your colleagues want to give me a call.