So, looking at it from a real zerg's perspective here, not just 'us playing as zerg' we can see that such a thing would be possible in the game itself.
This is a unique opportunity. The infesteds we got before, were forcibly turned, or subtly turned against their will while living amongst other humans through spores. These were necessary calculated militaristic actions. We were attacking an enemy.
Now, out of nowhere, these people most likely view us as their saviours, they saw the devastation of the rest of their planet. To calm them, an overlord most likely showed them Kingston attacking us around the same timeframe. We claim no knowledge of the hellspawn that started the invasion, only that he broke open a hole in Brontes' defenses, and we did what any rational commander would have done with this opportunity. We looked similar enough that the new menace did not immediately attack us.
They naturally fear us. They fear our might. But they are also thankful for getting away. At first apprehensive of their new master, they soon realise they are not mere playthings, more like valued 'pets.' Pets which can communicate their needs and desires. And so these are grown for them. They live as normally as possible in their biodome, and the cerebrate realises, this is not a learning opportunity it would have had the Overmind been around. It can observe them unhindered, like a child with an ant farm. Here, let us see what happens when you place a sugar cube here. Or a miniature weight here. Oh! They pick it up! They move it around! Joyous! We could always peek into their minds once infested, but it wasn't the same. And perhaps if they see how much these ants enjoy their captivity, others will wish to join too, of their own free will. After all, our primary mutation was a focus on innovation all-round, and voluntary infestation would be a truly novel approach to enveloping the universe.
bam, there's the canon hook.