I found this cool site for you that you can read up on this topic....
"Combining breast and bottle feeding
Many mothers do manage to combine feeding by breast and bottle - this is also known as 'mixed feeding'. Women who return to work, or know they're going to be separated from their baby for a length of time, may choose to use a combination of breastmilk and formula.
Some women express breastmilk so their baby can continue having breastmilk when they're not together. Breastmilk is made in direct response to your baby feeding - giving formula alongside breastfeeding could affect your breastmilk supply, but it's possible to combine successfully.
Speak to your health visitor or a breastfeeding counsellor about how best to manage this, so you can maintain breastfeeding.
* Wait until breastfeeding is well established, so formula has less of an effect on your breastfeeding - for most mothers a good time to do this is after five to six weeks
* Waiting also reduces the chance your baby ends up preferring the bottle - babies who are happy breastfeeders seem less likely to end up being confused between the different sucking methods at breast and bottle
* If you're giving formula at the same feed as breastfeeding, breastfeed first to keep up your supply, if that's what you want
http://www.bbc.co.uk/parenting/your_kids/b
abies_feeding.shtml