How to replace the pattern of passing callbacks with promises?

Question

I'm using Bluebird as a promise library and I have the following callback pattern that I would like to use promises for.

var func = function(err, data) {
    if(err) { // do this with err }
    else { // do this with data }
}

Desired calling pattern:

func.resolve(data);
func.reject(err);

How would I modify the original function definition to support the desired calling pattern?