Shamanism and Buddhism equally value that truth is in direct experience. As Ayya Khema states, "Wisdom only comes from the understood experience and from nothing else."
Both spiritual paths give access to unseen realms, and maintain that at a certain level of skill, the practitioner cannot go back to his or her previously mundane consciousness, and wouldn't want to.
In my experience, Shamanism and Buddhism differ in several significant ways. In Shamanism is the view that everything that is--is alive. While in Buddhism everything that is--is composed of mere elements devoid of self.
Shamanism places the animal and plant realms above the human one because humans are the only people that do not know, while all the other-than-human people do.
Buddhism places the animal realm below the human realm because to be human is to be ethical. Being innocent is not automatically virtuous; one must be capable of violating moral behavior and choose not to.
Perhaps the most pronounced difference is that the practice of Shamanism may lead toward manipulation, the bending of another to one's own will.
The practice of Buddhism leads toward virtuous behavior; moral discipline is the first requirement leading to inner calm. The Four Divine Abodes (Brahma Vihara) are Loving-Kindness--benevolence towards others; Compassion--motivation to relieve the suffering of others; Sympathetic Joy (Mudita)--happiness at other's good fortune; Equanimity--being undisturbed by phenomena.
Given my understanding of this difference between Shamanism and Buddhism, while a Shaman can be a Buddhist, I'm not sure a Buddhist would want to be a Shaman.