This isn't a career for the faint-hearted. So this belatedly brings us to what you asked about, now with the proper background for my answer. Earthtrust accepts a few people as interns, or as bootstrappers, sometimes a combination of these two. Generally the interns are folks who have just graduated, who can afford to move temporarily to Hawaii. Most of it is office work under a program director.
Some universities give course credit.
Windward Oahu is a nice place. The volunteer bootstrappers likewise need to have enough money to survive for as long as possible on windward oahu, and also get the intangible benefits of living in Hawaii, such as paying too much for groceries and rental space. If we judge them to be serious and non-flaky, we treat them pretty much as free labor for a time, until it seems they're well enough versed in reality to be let loose on the world. So - what you have here is one possibility for an environmental career. It is difficult.