I have asked this before, but posting again (about fed baby) ?

My almost 9 month old has never had formula. She does take an occasional bottle of milk. I do not pump regularly as I rarely leave her. I do keep bottles of frozen milk in case something comes up. Sometimes it is hard (not complaining, just stating a fact) to plan the rare date or girls night out. I have to make sure I have milk thawing, pump ahead of time, etc. I never know how much to take out, etc. I know you can thaw it under warm water right before feeding, but having it ready makes it easier on Dad or the sitter, and eases my mind that it is going to be easier on them. My question is, is it okay to start giving her an occasional bottle of formula? I know she may not like it, but I would wean her onto it slowly. Maybe putting a bit in her cereal or in a sippy. Is this okay, and is it wrong for me to want to do this? I plan on nursing her until 18 months old, this would just be a once in a while thing.....What do you other mommys think? I hope I get some answers this time!! Thanks!!! and be honest!!!