Last night I was out of town and needed to get gas but had forgotten my debit card. Didn't have a large amount of cash so I had to get a partial fill.
To make matters worse, I forgot to write down my odometer reading! Is it possible to add a fuel up without it? I realize it wouldn't be usable towards my stats but it would be nice to have the total litres and amount spent on record.
No, sorry, if you're using odometer tracking there's no way to add a fuel-up without an odometer reading. You might be able to retrace your steps with something like Google Maps to estimate what your odometer was at when you filled up.
Fill up again. Add the two amounts of gas volume (liters) you bought. Add the amount you spent together. Use the odometer reading from the second fillup. When you go to enter the info, divide the total cost by the total liters to get your per liter price, then enter the data. You will be accurate for your L per 100k, and you won't have a missed fillup.