How do I add stars to genes?

You don't!

OK, so the title was just mean, but it got you here, so let's talk about it.

The number of stars a gene has is like its Rarity. Like if you were to get an Orange Weapon in a game versus a Blue Weapon. You can upgrade that weapon until you max it out, but you can't change its color.

The number of stars a gene has is the same as the colors of those other weapons. The stars are innate to the weapon and while the gene can be fused or combined to make something else, it will still only be a one star gene.