Manually create A/B partitions

I have a Xiaomi Mi 6X which is the Chinese version of A2. There is a way to convert it into A2, but OTA updates won't work because 6X has only A partition. I'm not sure, but I've read, it is possible to create/delete partitions on an Android device. So I want to create A/B partitions manually, but I have no idea how it looks like.