It doesn't seem completely implausible, especially if the virus were artificially created so did not need to evolve that behavior via random mutation or gene acquisition.
Viruses by their nature make use of the host cell as part of their life cycle and often interact with its RNA and DNA, not always destructively.
There is much evidence that "sideways" transmission of genes between species via viruses was quite significant in early life on Earth and is still happening. Also other organisms can make use of genetic information from others in some circumstances,with bacteria sometimes doing this to effectively "share" resistance techniques.
See http://en.wikipedia.org/wiki/Horizontal_gene_transfer for a good starting point on the science that the plot idea was probably based upon. It almost certainly doesn't happen naturally IMO (though nature has done some other seemingly unlikely things, so you never know!) but the parts of the required mechanism exist in nature meaning it is not inconceivable that an expert enough intelligence could manufacture a virus that operated in such a manner.