Maybe, but you need to figure some things out first.
1) Is it a sewn-through bag or does it have baffles? Few 40 degree bags are baffled. Make sure. If it is sewn through, you would be better off with another bag.
2) How stuffed is the bag now? When laid out and given a chance to fully loft, does it puff up between the baffles? If it is already fully lofted there is nothing you can do to improve its temperature rating. If the bag is underfilled, you can improve the comfort range slightly. However, you will not get the bag down to 25 degrees no matter what. There is not enough slack in the system to double the loft, and that's about what it would take.
In the slim event that you actually decide to try this, most people who make down gear just go by feel. A handfull of down is not very much and you can just stuff here and there, moving it around as needed until it feels right.
Best advice: watch the after-Christmas sales or consider Mike Barney's idea of double bagging.