I THINK you can do this by creating the signing key with the "sign only" option and then adding an encryption subkey with the settings you want afterwards.
However I would STRONGLY advise against generating 1024 bit RSA or DSA keys unless you have absolutely no other option. The spook can probablly crack them now and they are likely to get further weakened over time both by improvements in the mathematical algorithms used to crack them and by increases in computing power.
If your correspondent really is running a version of gnupg so ancient it can't handle a 4096/4096 RSA/RSA key then you should really tell them to upgrade.