First of all, you will need a larger boiling kettle. If you are extract brewing, it is not necessary to boil the entire ten gallons. You can double the ingredients called for in a recipe, but you will still need to boil a larger amount of wort, at least 5 gallons; 7-7.5 gallons would produce a better brew.
Then, simply top off the fermenter to ten gallons with water. Of course, you will also need additional carboys for secondary fermentation. Don't forget more bottles or an additional keg.