First off, are you planning on weaning off BM at this point? If you want to continue to BF, I would try to express more, even though I know it is difficult.
If you want to move to formula, you can try other brands, as they all smell and taste different, and you can add BM to the formula to make it more appealing.
I found that the swapover time, while DS was getting both BM and Formula, he was very unsettled, and didn't want either the bottle of the breast. He was much happier when it was one or the other full time.
I would not consider adding anything else to the bottle. You could have breakfast cereal or rice cereal with formula for meals to help them get used to the taste as well.
If this is only for one night, I would try to stock up small feeds of BM beforehand, at 8 months it is more the soothing feeling of sucking that they want, rather than a full feed.