Mixing Wella T18 with dye?

I've been lightening my hair over a few weeks now and after a gentle bleach bath that didn't do much, I tried toning it with Wella T18, and it left me with a light yellow/beige shade with a few brassy bits. I'm super stoked that I did that much, as I was worried my hair would be too dark for it to take properly, but I still want it lighter and cooler. Preferably I want full on frosty platinum Elsa hair, but I know my hair is stained from previous dyes and I don't think more bleaching will do much but further damaging my hair. However, I have had some success in the past with using a cold, light blonde hi-lift box dye to nudge stubborn stained hair lighter, and so I was thinking of trying it just to see if I can get it any lighter at this point. I'd need to tone it anyway and I've found purple shampoo's and conditioners just doesn't take well on my hair, despite leaving them in for ages.

However, being a savvy shopper I bought two packs of T18 and developer only for one, and as I live in Norway I'd have to order online again to get some more. However, there is developer/activator in the box dyes, which tend to not really get cold enough for my taste anyway, so what if I slipped a little bit of T18 in the dye mix? Just to get it even colder.

I know that might be a really dumb idea and I need to give my hair a bit of rest for now anyway while I think about it. I know there's a risk when you mix different brands together as well but the Wella dyes that are available here on my cold little Northern rock aren't light nor cold enough for my taste.

I'll add a pic of my hair's shade once it's dry and the daylight returns.