Besides any built-in methods within your software, I've used two different modeling methods:
1. Equivalent X-brace.
What you do is create a series of multiple X-braces throughout the diaphragm plane. These braces are simply modeled as generic members that only take axial load and are weightless.
The amount of X braces depend on your model size but similar to finite elements, the more the better.
Once you get the braces in place the key here is to set the cross sectional area of all the braces to mimic the behavior/stiffness of the metal deck diaphragm.
First you need to do some hand calculations to find out, for some given lateral diaphragm load, what the deck deflection will be. Vulcraft or other deck manufacturers (or in the US - the SDI manual) will provide stiffness parameters for various decks and fastenings. With the G' value you can calculate the deflection at the midspan of the diaphragm.
Now with that delta deflection, go to your model, apply the same lateral diaphragm force, and vary the area of the X-braces to achieve a similar deflection. Once set, you can now continue modeling the framing and size the members. Once you design the diaphragm, you need to back-check that the fastenings and deck thickness you need for strength is consistent with the X-bracing stiffness - you may need to iterate a bit.
You also need to be very aware of the discreet nature of the individual X-braces and how they might affect the framing members. An end of a brace may indicate to automated features within your software that there is a brace point, etc. and affect design routines. Just set up the braces to only act as braces and not members taking other out-of-plane forces or affecting member sizing routines.
2. Equivalent plate elements
Similar to above except instead of using discreet X-braces you use finite elements that only take in-plane forces. Vary the element thicknesses to get a matching deflection to that determined by hand calculations.
