I read somewhere that it's okay to add fresh milk to frozen milk AS LONG AS you chill the fresh milk first. You don't want to mix body-temperature milk into the frozen milk because it could thaw it a little.
Also, as jeno said, I would not recommend freezing too large of quantities. Once you defrost it, you have to use it within 24 hours. I personally freeze 3.5 ounces at a time (whenever I am lucky to have extra milk), so that it is just one bottle to defrost and use right away. (DS is eating 3.5 ounces at a time right now.)
Mommy to Alex, born 2/27/11