What you're looking to do is called [*high gravity brewing*](http://www.craftbeer.com/craft-beer-muses/high-gravity-beer-big-risk-bigger-reward). This technique is oft employed by macro brewers to produce more beer with less fermenter space. They dilute after fermentation is complete.

**Some Considerations**

 1. Yeast Health: Your yeast will be exposed to a higher osmotic pressure because of the higher sugar load. This is stressful to them if you haven't properly pitched the right number of yeast. Consider asking three people to empty a buffet designed to feed fifty. Pitching rates are based on degrees Plato, so a higher gravity wort will require more yeast, of a strain that can handle the higher load.
 2. Colour: Colour, despite what one might think, does not change linearly with dilution, so your colour may not be what you expect when you dilute.
 3. Bitterness: When you calculated your initial post-boil volume, your bitterness was also based on this volume. If you dilute, you will lower your IBU proportionately. 
 4. ***Sanitation***: This should go without saying, but don't ruin your beer by diluting with straight tap water. Boil the water first, cool it down quickly, make sure you're doing things in a sanitary manner.
 5. Ion content and Water profile: When diluting high gravity worts / beers, you'll want to make sure your water profile is similar to that of the beer. This is less of a concern on the homebrew scale, but all the same, if you filtered before filter now etc.