as far as I know adding distribution is not supported, you could write your own custom distribution omnigraph node, or you could use a pre-computed list of random values and use that with choice:
import random
import omni.replicator.core as rep
NUM_VALUES = 100
custom_random_values = [(0, 0, val) for _ in range(NUM_VALUES) for val in [random.uniform(-2, 2) + random.choice([-90, 0, 90])]]
# rotation=rep.distribution.choice(custom_random_values)