You could enter an exact rating curve using a special outlet, but a trapezoidal weir or custom weir should be pretty close. Even though these are assumed to be symetrical, they can be used to approximate an asymetrical situation with a minimal difference in the discharge or head. If in doubt, try both and compare the results.
Keep in mind that HydroCad is not set up to do FHWA inlet capture calcs. Bypass flow is largely a function of gutter spread vs. width of the inlet. If this is what you are after, you may need to do some of these calcs outside of HydroCad.
