L.
I'm pretty sure you add the breastmilk after thawing, but I can't really remember why. wholesomebabyfood.com is a GREAT site with all kinds of info about making baby food, and they are endorsed by everybody. ;-)
I agree with Marda though--stuff like bananas and cereal you can easily mash/mix on the spot. I only mass produced and froze stuff that I cooked in big batches, like winter squash or baby dahl. :)
Like Jane though, I didn't usually mix with breastmilk--the quantity was so small that I usually just added a bit of water when I needed to thin something. My babe chugged plenty of breastmilk each day, so I didn't worry about it too much and saved some time and effort I could better spend elsewhere. If I pumped anyway (for work or whatever), I guess I would have used it more consistently though.
Also, both my babes had trouble with constipation when adding solids. Just be on the lookout since rice and bananas can both be binding. :-/