Is there a way to simulate adding H2 (generated by an electrolyzer) to a natural gas powered generator?

HOMER does not have the capability to model a generator co-fired with electrolyzed hydrogen.

I can think of a couple of short-term workarounds that you could use if you are willing to do a bit of post-processing of the results.  If the ratio of hydrogen to natural gas would remain constant throughout the operating range of the generator, then you might want to model the generator as a pure hydrogen generator, and figure out the natural gas consumption afterwards.

For example, say you want to assume the generator will always operate on 33% hydrogen and 67% natural gas.  Then you could model the generator as if it consumes hydrogen only, at a very high efficiency.  If it ends up consuming X kg/yr of hydrogen, then you know you will need 2X kg/yr of natural gas as well.

More likely, you will want the ratio of hydrogen to natural gas to vary in time, according to the availability of the hydrogen.  If that's the case, you might be able to model the co-fired generator as two separate

generators: one hydrogen and one natural gas.  The natural gas generator would fill in for the hydrogen generator during times of insufficient hydrogen supply.  You would have to be careful when specifying the properties of the generators such as minimum load ratios and capital and O&M costs, so that the calculations work out as accurately as possible.  You don't want to double-count O&M costs, for example.  I haven't tried this trick so I'm not sure that it would work, but it seems possible.