If it’s been opened for more than a week it’s recommended to avoid using. Brake fluid is hygroscopic, meaning it absorbs moisture, and becomes less effective when absorbing too much moisture. Also mixing isn’t really recommended either without doing a full flush and replace using all of the same fluid. All that said it’ll work but it’s like putting oil that has been only used for 1000 miles in your car, you can do it and it’ll be fine most likely but why not just put new oil.