I don't know how it works to add an infant later on. I would imagine you just need to call their ticket/sales office and add from there.

As for the bassinet, they put in a request. In my case, the infant was booked with me and DW. However, I remember having to ask them for a bassinet. First time I was told it would be there for sure. Second time I was told its based on availability, then a third time I was told to make to ask for it at the airport.

Ah Turkish.