In your signature block I see you've got a âWOW variantâ made with WGJ + apple juice.
That isn't a WOW variant, it is often referred to as Basic White, and if anything WOW is a variant of Basic White.
Anyway, that's what you need, start another Basic White and when fermentation has quietened down so that you think it is safe to top up your DJ, add your elderflower cordial, perhaps in a couple of stages over 2-3 days.
Are there any preservatives listed on the label?
If so, add 100ml a day for 5 days and your working yeasties will probably ignore them.