There isn't an update necessary for this, as this feature already exists for games whose publishers decide to do that for. I do not own Red Dead Redemption 2, but if you look at a title like Borderlands 3, you can see that all of its add-ons are available for separate purchase and that the editions simply bundle these together at lower prices.
In short, this is more of a Rockstar Games problem than a Stadia problem. The store platform should already allow what you are requesting.
@MrPinkCow Unfortunately there's no way to upgrade editions for RDR2. And this is not a Stadia specific issue, as it's actually the same case on other platforms as well. If you haven't played more than 2 hours, and owning it less than 2 weeks, you can try getting a refund, and then re-purchase the other editions. Or contact support directly and explain the situation, maybe they'll be able to make an exception.