This is the nightmare thread that will give you the information you need:

1. Some reasonably positive experiences: Adding Infant to Business Class Redemption Ticket
2. Some less positive examples with awards: How do I add an infant to a QR flight booked at Asia Miles

Based on my personal experience, even when trying to add a cash fare to a cash ticket issued by QR after birth, they refused for my second child (this is 2024), whilst in 2022 they were OK with it for my first (it was just expensive as booking a lot of one-way tickets, hence why there are conflicting reports - there was a policy change).

So overall, QR is not a good option for your plans. You should choose another airline that actually cares and is willing to help (I would call out AA, BA, Cathay as all being very reasonable in this regard). I ended up in stupid situations like my wife being denied boarding by QR, and having to buy entirely separate tickets for her plus the infant (that by the way they wouldn't sell at the airport, but had to be booked in advance via the website and entirely unlinked to my own ticket, I just didn't see the need to cancel the other tickets they refused to change :P). Whereas CX was quite happy to add an infant to a QR operated, but CX sold codeshare.

Note: Once the kid is born, I have found the website perfectly reasonable to book the tickets for either cash or Avios for infants, it is just the mid-journey/post ticket addition that is the problem. Therefore I have a feeling that if you used a "Vaguely right" date and name it could work - but how many people want to set the name of their kid before birth (or have a name change to accommodate QR ticketing...)