There is no special way to ask this, to my knowledge. You can just tell your mangager manager that you are underutilized at the moment and ask if there is something you could work on. If they don't have something, you can ask if they mind if you offer to help someone else. I would recommend doing this privately.
I stress checking with your manager first from my own personal experience. I once was in a situation where my duties didn't take 40 hours per week for a brief period. The group I developed products for was over extended. I thought helping this group would help me learn to do my job better, as well as allow them to get more done. Since it was also overseen by my manager, I went to him and offered to help them out. Surprisingly (to me) he said no; then he explained that the nature of that group's work was such that I could easily get pulled into helping them full time and he couldn't afford to lose me in my product development role. Furthermore, it would take some time to become familiar enough with that group's work to become productive. With approval of new products expected shortly, he wanted me to start that as soon as possible. He was concerncedconcerned that if I started working with the other group, and then approval of the new products was received, I would either leave work undone with the other group or delay start on the new products.