I believe you can do it with pywallet:https://github.com/joric/pywallet
I've used it to import private keys into an existing wallet.
You can use it to dump the keys from the first wallet, then import the dumped keys into the second wallet.
It doesn't appear to have been updated to work with 0.4.0 encrypted wallets yet, so use with caution.
i installed pywallet and it's pretty good at what it does.
but the fundamental part it lacks is importing multiple
keys... i mean i kind of would have expected it to be able to read it's own 'dump' output, but no, you have to do them one at a time which unfortunately renders the feature useless to me in all practicality.