Sure, but keep in mind a few things.

1. National is serviced by Europcar, so any status you have with National won't mean anything over there.
2. There are usually hefty drop fees to one way between European countries, because unlike the US, each car has to be repatriated with its own home country. It's not like the US where we have cars registered in different states and it doesn't matter where the car ends up when it is done with fleet service.
3. Switzerland requires all cars (regardless of origin or status as rentals) to have a road permit vignette, which is not cheap (40 CHF iirc). You would have to buy one of these. Get pulled over without one and there is a huge fine, and there are often checkpoints along Swiss highways to check for them.

Last year when I did this exact itinerary, we rented one way to Konstanz, on the German side of the German-Swiss border. Paid 10 Euros for a cab to the train station and then another 10 Euros for a train ticket to ZRH.

If you still need a car in Switzerland, your best bet is to pick something up on the Swiss side of the border.