You can split a PNR but not merge/add to a PNR. What is the reason you want the child to be on the same PNR as an adult? If it's so they are not considered an unaccompanied minor, I don't expect there to be a problem. When you call to split your PNR, the phone agent can then link the child's PNR with yours on the outbound and with your wife's on the return.
https://saleslink.aa.com/en-US/resou...arate-pnr.html