This is a portmanteau of "volunteered" and "told" and probably has its origins as slang in the U.S. military.
Most anyone who has worked for an organization of any size is likely familiar with the concept, if not the word.
Imagine that you're busy working on a full plate of projects when your supervisor knocks on your cube wall: "Hey there, we've got some visitors from England coming next Tuesday, and the vice president was looking for volunteers to show them around the facility. I told them you'd do it. Hope you don't mind!"
Yep ... you just got voluntold!
The precise duties involved after being voluntold can range from pleasant to neutral to stressful, but the key aspects are that this is a "volunteer" job that you didn't ask for and can't turn down without causing trouble for yourself.
To be fair, this is not merely a corporate phenomenon; people are regularly voluntold in families, too: "Honey, I signed us both up to go give blood this Saturday!"