Another way of doing this (i.e. besides creating a new character and playing it offline in coop on your own) is to just finish some missions, but don't collect the rewards yet! Once you have about 4 or 5 finished, go to the area where you're supposed to collect them, and quit the game to the main menu.
Then, start a game in off-line coop, pick your own profile and pick any character with the other (can be a level 1, if you haven't got another). Go cash in those missions, and quit the game again. Repeat 3 to 4 times until you have 15 in total.