can you add a due date to a specific phase rather than the whole card?

Using some sort of workaround, yes: you might be interested in either:

• - setting a maximum time (SLA) for a specific phase in the phase settings (advantage: easy, not really a workaround. Disadvantage: not card specific/each card has same maximum time), or
• - resetting the due date upon moving the card between phases by using an automation, possibly in combination with some field conditionals (advantage: highly customizable, disadvantage: some setup work needed)