There was foreshadowing to her existance. it was just extremely subtle. Basically all of the Zoldyck children's names connect like IlluMilluKilluAlluKalluto. There's always a "lu" in the name that seperates the first and last part, and Kalluto is introduced before Alluka, and if you realize that his name doesn't connect with Killua's you could assume that there is another inbetween. By taking the "a" in Killua and the "ka" in Kalluto and putting a "lu" in the center you can even name Alluka before her introduction. I'm not saying that anyone who disn't find this out is stupid, I'm just pointing out that Togashi had to have planned Alluka atleast ever since Kalluto was named.