No, in general re-using keys is not advisable. Even if you would have a secure token then it would be better to have multiple keys stored in it.
In general the level of security required for certificate authorities is higher than the security required for personal use of a key. If you later want to your CA key within a smart card or HSM then it should also be able to perform your PGP operations. This is not that likely.
You will have the same issue if you want to revoke your CA or private key; keeping a key operational after revoking it for one particular use is not good practice. So a revocation of the key for one use should (and hopefully would) imply revoking it for the other use as well. One good reason of revocation would be to use a key of a larger size for one of the two protocols.
Then there is the problem that some protocols may interact with other protocols, as CodesInChaos already stipulated. It may also be that some protocols are less safe than others. For instance, decryption with an RSA private key is generally considered less secure than performing a signature generation (in general, an attacker has more influence on the ciphertext). So it is common practice to separate encryption and signing keys - see e.g. the key usage attributes in X5.09v3 certificates.
Maybe you could deploy the same access mechanism for the keys though.
Almost forgot: yes, it is possible. RSA keys are generally the same for most protocols - it could be that it is not advisable to have a very short public exponent (say, the value 2 or 3) for some protocols, but that's about it.