HELP.Can I Add Water After Boil?

Just completed first all grain today. After boil seemed to be shy of 5 gallons. Using a 6 gallon fermenter beer level is only 3/4 way up. Can i boil some more water and cool and top off fermenter?

Go ahead and top off...but take a NEW hydrometer reading afterwords. Your OG may be off from your original plan a bit, so you'll want to know.

You should worry less about your final volume, and drive what you do more from your final gravity - because that is what will determine the style / nature of your beer.
Many brewers adjust the boil time to concentrate the wort or add water to hit the gravity desired.
If you don't have the volume you expected at the desired gravity, then you must adjust either:
Your boil vigour. If you are just rolling, that's enough. You may be evaporating more than you need to. If you are not aggressively boiling or losing a whole lot when transferring then,
Your efficiency rating in your recipe to compensate (with more grain), so that when you top up to your volume, you are not overly diluting your gravity.

Cheers.