I am making low cal pumpkin muffins using a box of spice cake mix combined with a can of pumpkin. Can I add a little baking powder to make them rise up and be a little bigger?
You can certainly try adding more baking powder, but there are a few things you might want to consider before you try that.
First, you don't say whether you are making the cake mix according to the instructions on the box, or modifying it in any way. According to the USDA nutritional analysis for canned pumpkin, one cup of pumpkin contains around 7.5 fluid ounces of water. By weight, canned pumpkin is over 91% water. If you are using canned pumpkin pie mix, things are slightly better. A cup of that contains only 6.5 fluid ounces of water. That is still 71.5% water by weight, though.
For cakes or muffins, if the batter is too thin, then the bubbles released by the baking powder, or other leavening, may rise to the surface and escape before the batter starts to set and is able to trap the bubbles. If you are not compensating for the water in the canned pumpkin, that may make your batter too thin. Since the water is trapped inside the pumpkin, the batter may look good going into the oven, but still be too wet.
To remove some of the excess water from canned pumpkin, the folks at America's Test Kitchen line a rimmed baking sheet with a few layers of paper towel, spread the pumpkin evenly over that and then cover with another layer of towel. Pat lightly to press the top layer of paper towel into contact with the pumpkin. Peel off the top layer of paper towel and then roll the pumpkin off the bottom paper towel as if you were making a jelly roll. This technique will remove over a quarter cup, or about 1/3 of the water.
You could also correct the cake recipe by reducing the other liquids used.
On the other hand, if the batter is too thick, then it is more difficult for the leavener to work, so if you are only using the mix and canned pumpkin, you may need to add a bit more liquid. Either way, you will likely need to experiment a bit to get the desired result.
So, back to the original question, yes, you can try adding more baking powder, too. Start with maybe 1/2 teaspoon or so, and see what happens. Too much baking pwder, though can cause your muffins to rise too quickly, resulting in them splitting open or collapsing. If that happens, reduce the amount of added baking powder and try again.