Hello amclotfelter
Yes, it is possible to make a seat reservation on the EC Train. You can do it on the phone with a credit card 0848 44 66 88 (CHF0.08/Min.) or at any ticket counter in Switzerland.
However, if your travel plans allow it, I recommend you rather switch to an IC connection, because the EuroCity trains are often booked all the way to Italy and are therefore very busy.
I wish you an excellent afternoon. Regards Rahel