When racking from a primary fermenter to a secondary vessel, you will leave behind a non-trivial amount of "stuff" so the volume in the secondary will be less than the volume in the primary. If you start with five gallons in the fermenter you won't have five gallons left to bottle, but it isn't any more concentrated than when you started.
If your OG and FG were about where you expected them to be, you probably should leave it alone. Otherwise, you're just diluting your beer. Quality over quantity!
But yes, if you want to top up, you can boil and cool some water and gently add it without causing harm (other than dilution).