Tuplets on 5 beats

I am in a 6/4 time signature and wanted to write 5 beats in a measure as 15ths tuplets (with half notes marking), I had 5/4 that i could mark with 5 quarter notes divided into triplets or with multiples. But if i want to write that as a string of 15 notes it doesn't allow me since you can only put a tuplet on a note that can be notes as a single beat of some length, but not tied to another note. For example, a dotted note or a double dotted note can be separated, but there's no way of marking 5 beats with a single symbol.