Hello Ekir75
You are talking about the Swiss Travel Pass.
It is valid as far as Basel Badischer Bahnhof, short Basel Bad.
From there you will need a ticket which you can purchase from SBB or Deutsche Bahn (DB). And yes, it is possible to combine the two tickets.
The seat reservation is optional and to be booked separately; you can book it for the whole trip from here:
https://int.bahn.de/en/offers/seat-reservation
Kind regards,
Roland