Gene modding a population into a "hive drone" would kill them if they weren't already in a hivemind government, but probably not instantly. Now, to have a hivemind government you need to have picked the three point Hivemind ethos at empire creation. Which simply turning all of your people into hive drones would not accomplish after the fact, so just by itself gene modding would be a creative way to suicide your people.
Since this doesn't happen instantly though you may have another move. Now, to change your ethos all you need is for a faction to get 20% "support" in your empire. This might mean something as simple as 20% of your population need to be following an ethos before you can embrace a faction. If so, the act of genemoding would change your populations ethos to "hivemind" and so it's conceivable within the given system that you would be able to spend influence to "embrace" the Hivemind faction moving your empire ethos one step in the direction of Hivemind and thus saving the lives of your gene modded drones.
Realistically, this is going to be locked out and would need to be modded in. Also, there's the issue of Hivemind being created as a 3 point Ethos so that you can be locked out of the Ethos and faction system, if it takes 3 embracings of the Hivemind faction, then the 20 year cooldown on being able to embrace a faction is likely to cause the first two groups of populations you mod, or 20% of your empires population, to die horrible deaths each time, their sacrifice a horrible necessity to fulfill your dream of unity.
Edit. Hivemind is a faction in the government screen in the Developer Diary 61. It's that icon that has two empty hexes on top of a single full hex. I still think it's likely to be locked out, that it's in there as part of a beta build and will be edited out, now that hive mechanics are fleshed out enough for a hive faction in non-hive governments to be impossible.
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