What you're looking to do is called high gravity brewing. This technique is oft employed by macro brewers to produce more beer with less fermenter space. They dilute after fermentation is complete.
Some Useful Resources:
Some Considerations
- 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.
- 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.
- 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.
- 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.
- 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.