can you mix breastmilk and formula together?

Ds2 is my third child and we are breastfeeding. I do however give him a bottle of ebm as his last feed before bed. I had issues with my ds never wanting to take a bottle and was only ever breastfed which made life difficult when I had things on I couldn't or shouldn't take her to.
So I made the decision to give ds2 a bottle at an earlier age and he takes it fine. I bought those prima type packs of formula as a back up just incase I ran out of ebm.
Just wondering if you can mix the two together. I don't want to give him purely formula incase it upsets him!
I have to leave him for a full day in September so I am wanting to trial formula just incase my supply is low!

I had huge problems getting DD2 to take a bottle and I was told by the CHN to try mixing formula and EBM together, starting with almost all EBM and then gradually swapping the proportions.