Yes, it's possible.. It really depends on what you want to do with it, is it just an app that you access? like weatherbug? if so, then yes, you'll still be able to access..
If you want to actually incorporate it into a theme that would have to be created from scratch, if you aren't familiar with creating themes, you'll have to ask someone to create one, if there's not one already created.
It's an app from the Appstore called Megaweather, I like how the 6 Day forcast looks and the current forcast parts but looking in the Megaweather folder, it looks totally different than a weather widget so I have no idea on how to even put it in.