Yes it's possible to include a negative amount into a new transaction, as for the maximum amount allowed depends how the lender books the vehicle in question you are buying, the dealer will have answers.

However adding in your case about $5000.negative into a new loan is red flags BIG TIME as you will be in a much worse negative situation should you want to trade or sell prior to having paid loan in full.

so would i be paying a monthly payment on $11000
the $7000 im upside down plus the $5000 being rolled into the new loan. or will i be paying a monthly payment on $31000 the $26000 plus the $5000