Can Volcanic Eruptions Help Combat Global Warming in the Long Run?
The subject you are looking for is called climate engineering. In general, changing atmospheric composition in order to achieve a cooling effect that counteracts climate change is conceivable.

Whether triggering volcanic eruptions is the most feasible approach (or possible at all) is of course a completely different issue. From my understanding of volcanoes, you would really need massive eruptions with large plume-heights to achieve long-term impact.

Also note that we would need to sustain the resulting atmospheric change over a long time. And the side effects of releasing large amounts of pollutants into the atmosphere need to be considered too.

For some more aspects read this answer to a related question.