It is impossible to get to the center of the earth using a volcano for the following reasons --
1) Lava is super hot, so if you go into a volcano and touch the lava, or get too close, you could get severely injured
2) The center of the earth is even hotter! It's possible for people to melt, so you would most likely melt if you were trying to get to the center of the earth.
Hope this helps! :D