There is a totally free plugin that not everyone knows exists. It's an old one, but it can make magic out of simple samples and loops. It's called PaulStretch.
As it name might reveal, it's a plugin that stretches sounds. This is no ordinary stretch though, it's using some advanced algorithm that is able to stretch sounds up to 10 000 times. Isn't this amazing? Yes it is!
Download this plugin over at their site. (It's free and open source which all producers love).
So when can you use this plugin? Well, I have a cool example that I'm using every now and then.
But first, do you have a cool short synth / pad / vocal sample that you really like? Good. Use that.
The Stretch time slider will let you set how much you want the sample to be expanded in time. Use as much or as little as you need, but remember that a longer file will be bigger and will take longer to export. You probably won't need a 1 hour sample.
The play and pause buttons in the down left corner lets you listen to how it sounds before exporting.
Now you have your long sweet pad / synth / vocal or whatever. This can now be used in your tracks, and will definitely help you fill out those frequencies. Great. Bye.