From what I've found searching around it seemed like the recipe for making sunflowers in previous versions of Ex Nihilo (Not omnia though) was by rightclicking a witchwater-barrel with a dandelion. But in Omnia this recipe doesn't seem to exist. I'm currently playing Forever Stranded, so I'm using Ex Nihilo Omnia version 1.2.6 (MC version 1.10.2).

From the wikipedia page regarding flowers, there are no way to get sunflowers otherwise as they are only generated naturally in sunflower plains biomes and you can't get them by bonemealing even after you change a chunk to said biome.

Now, it ain't a huge problem, sunflowers are only really needed for a few Pam's Harvestcraft recipes in the pack I'm playing.

I hope you'll consider adding a way to get it, doesn't have to be dandelion -> witchwater -> sunflower, that was just a suggestion.