Absolutely! I've used conditioner to lighten my hair dyes before and it works like a charm. :) Since Splat dyes come in a bottle, I would recommend pouring it into a bowl in order to mix it easier. Then, add white conditioner a little at a time, mix, and repeat until you achieve your desired shade. The dye should still work just as well, but try leaving it in overnight anyways. Also, be warned that if you want a very light baby pink, your hair must first be bleached almost white in order for the colour to take properly.
However, In my experience, Splat Luscious Raspberries is a very pinkish red colour, so if you just apply it normally it will eventually fade into a nice pink. Splat also has a pink dye (called Pink Fetish), so if you go back to where you bought your dye from you should be able to find it.
Best of luck!
Lots of experience with hair dye :3