I think that OP means with “round trip” what is usually also referred to as “return”. Ie not “Rundfahrt” but “hin und zurück”.
Yes, you can buy a return ticket. Just indicate that when purchasing. A return ticket is however the price of two singles, so there is no savings, just some convenience.
Buying a “Rundfahrt”, eg Interlaken - Montreux - Bern - Interlaken is possible at the station. But for such trips you are probably best of just getting a day pass.