This is not a meteor question, but an icomoon question. The answer is that you don't add an icomoon set to font-awesome, rather you let them run together side by side.
What is meteor specific is how you can access those fonts, so they will need to be in your /public/
directory.
You can find an example of this through my meteor starter set. You would put your extracted icomoon fonts inside of that /assets/
directory. Then inside of /client/less/
(or sass if you use that) you would put your icomoon file as an import. Finally from your main less file you would import the file like @import "/less/icomoon.import.less";
at the top of your document.