That's not something that ROKU does, nor would Bell likely be open to it.
If you notice, Crave is only available on the web and on devices made by companies that Bell already has a relationship with on the mobile side of their business (Apple, Google, Samsung etc.).
Their incentive is to keep people from cutting the cord and keep their bottom line padded. Putting an app on the most popular Canadian platform for cord cutters and on a device that they don't get a cut of the sale of or that doesn't make their partners feel warm & fuzzy doesn't get them anything.