Is there an encyption scheme that combines additive homomorphism with ability to proxy re-encrypt? I've tried digging around on the Internet but haven't found anything conclusive on the topic.
The schemes in this paper are ElGamal type single-use unidirectional proxy re-encryption schemes.
Although by their construction they are multiplicatively homomorphic (they do not state it in the paper, but works analogously to ElGamal), you can use the same trick as in "exponential ElGamal" to make them additively homomorphic, i.e., represent the messages $m$ as powers of the generator $g$ of the target group ($G_2$ in their paper), i.e., as $g^m$ for messages in $Z_p$. Then, you are, however, limited to "small" message spaces as decrypting involves computing discrete logs in $G_2$. I do not know your application, but in many scenarios that should be sufficient.
As far as I know it has never been used in this context and there are no papers I am aware of that use additively homomorphic proxy re-encryption. There may also be other schemes, but this one should be a good starting point. As already stated by Reid, Gentry's scheme can also be used for that, but thats not practically efficient.