Unfortunately, I think your only option is to use a travel agent.
El Al is very peculiar among airlines in that while everybody else makes it really easy to price, reserve and buy just about anything an agent can do, El Al - though they supposedly want us to buy their tickets online and squeeze the agents - doesn't allow for any non-standard options while booking online.
Typical. Kind of like their website. And antiquated phone system. And pathetic IFE (where even the three planes that actually have VOD don't get content updates more than twice a year). Lets just say that although she is the flag carrier of nation that is a tech powerhouse, technology is not exactly El Al's strong point.