In fact, it's the only way you can be either.
Anarchism and capitalism have the same definition to me: the lack of political authority that gives a legalized means of initiating aggression against private property.
I believe the two are inextricable, and that you can't be one without the other.
You can't be a capitalist if you're also a socialist, say, of the George Bush variety, that "we had to go against free-market principles to save the free market", i.e., bail out industries with tax-money; and you can't be an anarchist if you want to be in the same business as the State is of stealing property from natural owners and redistributing it to others by some sort of egalitarian scheme.
What is "voluntary" must be conceived of with an ethic of property in mind. An ethic, that is, of private property, seeing one's bodily self-ownership and that of the scarce resources in which they have appropriated for themselves and their survival; a necessity of man's existence in this world, which he, endowed with reason, knows he must grapple with. Any outside interference, such as claims to his property, must be defended, as well as considered to be aggressive.
Anarchism, being anti-State, and capitalism being that of private ownership of resources, which of course the State must expropriate for its survival, contrary to man's situation, make a conjoining of the two – anarcho-capitalism – a compatible philosophical position.
I see no inherent contradiction; the position of anarcho-libertarianism, contrary to what the "anarcho"-socialist would assert, who are against the property ethic of capitalism, which likewise should be anarchism's position, of leaving natural owners of property free to exchange property titles to that which they have appropriated for themselves in accordance with the homesteading principle, is consistent.
Any sacrificing of one or the other, of violating the "anarcho" part in an anti-anarchist political-socialist manner or of violating the "capitalist", private property rights part in an anti-capitalist-economic manner, necessarily negates the other: socialists are not capitalists and anti-capitalists aren't anarchists.
I'm an anarcho-capitalist: political and economic liberty for all, based around property rights.