Is it possible to use a PGP key as a bitcoin address? Both bitcoin and encrypted emailing are based on public key cryptography. However, they use different implementations, so I assume that a public PGP key cannot be used as a bitcoin address directly.

But would it be possible to convert a public PGP key to a bitcoin address in such a way that it can be accessed only with the corresponding private PGP key (or something derived from it)?

If such an algorithm existed it would allow for sending coins to people without them knowing anything about Bitcoin, as long as they have a PGP key (e.g. for giving someone a little incentive to learn something about Bitcoin).
No, this is not possible.

Recent GPG versions do have support for Elliptic Curve crypto, with associated EC keys. The EC curves supported there are different from the one used in Bitcoin, though (they don't have the same size, so they certainly can't be isomorphic).