That would not be recommended. You might get FLEX channel operation, but you might not. You might also not get ANY operation, as they may not play nice together. If you want to avoid problems and potentially having to return what you've bought, buy a set that comes with matched sticks. Yes, it will cost you more, but you can always sell the ones you have now and avoid having to pull your hair out later.
Probably it will work, if you don't mind fiddling with the BIOS and configuration settings, but if it doesn't, which it might not, it's going to be a headache for you. And if it disables your dual channel operation because of the oddball stick, it might actually HURT performance rather than add to it.