Ah, no - you cannot do that in InstallAware, because of the advanced scripting it employs.
However you can execute setups, and in particular other existing MSI files, directly as part of an existing InstallAware installation. Just use the (Un)Install MSI Setup plug-in command in your setup script. Your main InstallAware installation will even capture and display the progress of the shelled-to MSI installation as part of your main setup progress.