There are adapters for Bay I to 49mm, 52mm, and maybe some other standard filter sizes. If you already have filters in those sizes this can be a good way to go.
The Bay III filters are worth something so either hold on to them for when you get a 2.8 Rolleiflex or look into selling them. In the meantime, the best thing you can get is a lens hood. The cheaper plastic hoods from Ebay are better than nothing.
Enjoy the 'Cord. The Xenar is a great lens. The Triotar is also fun, so be sure to try some shots wide open.