if it's an aftermarket neck designed to replace a strat neck then it will do that without any problems, all they've done is extend the fingerboard slightly so it overhangs the point where the neck joins the body, the dimensions of the neck remaining the same.
On a recent telecaster style guitar I built, the neck was fretted to a Gibson scale (24 + 3/4'

, but you could fit a standard Fender scale neck (25 + 1/2'

to it and it would intonate perfectly, as the bridge is placed on the body at a point optimum to both.
In your case the scale length will probably be exactly the same as a 21 fret neck but with an extra bit of fingerboard to hold the extra fret