One solution:
Get the pipe orifice formula from Handbook of Hydraulics, King and Brater, 1963, p.4-14

Q = Ca x sqrt [ 2g x ( p1/w - p2/w + v1^2/2g)]

Headloss, H = p1/w-p2/w

Rearrange and solve for H

H = [ (A/ca)^2 -1 ] x v1^2/2g = K x v12/2g

where K = (A/ca)^2 -1

From; 'A' (pipe area), 'c'(orifice coefficient), 'a'(orifice area), 'K' can be computed. Of course 'c' is a bit tricky and depends on the orifice shape and Reynolds number.

Insert the orifice into your EPANET data file as a very short pipe
Use L=0.01 ft. or 0.01 m (whatever)
Hazen-Williams C= 99
specify check valve
Coeff K- as computed above

I believe that you could also install it as a TCV also, but I prefer the short pipe.

I am assuming that you downloaded EPADATA and can enter you data with ease.

http://www.nps.gov/dsc/dsgncnstr/hydro.htm

Istvan