I've seen that you can add the [rollable] expression to magic items and the description appears as normal (meaning you only see what is in the quotation marks and not the entire expression). I haven't been able to find any notes on making aspects of the magic items (mainly damage rolls, really) rollable. Does anyone know?

Thanks!

That expression doesn't do anything on magic items, or anything other than monsters. It adds rollable buttons to monster stat blocks in the encounter builder and combat tracker.