I've had LH issue an infant ticket on a TK- stock award ticket but it was absolutely painful finding someone who could help

you'll definitely need to speak to El Al. My understanding of the process could be for them to create a booking for 1 A 1 inf. Then split the PNR and issue the infant ticket and cross reference the PNR with yours (cancelling the adult one they created)

you'll then pay 10% of the adult fare on sale today + any taxes

The issue I ran into was the flight was full so they couldn't price it this way. I eventually got hold of someone in the local LH office and after typing war and peace she managed to build and issue a ticket using the lowest published fare. So if you can I would try to speak to a city office or airport ticketing counter etc rather than call centre