What you are trying to do is not yet implemented in the stable version of rails so Michelle’s answer is the right one for now. But this feature will be implemented in rails 4 and is already available in the edge version as follows (according to this CHANGELOG):
$ rails generate migration AddImageableToProducts imageable:references{polymorphic}
Some shells may need {polymorphic} escaped with a \:
$ rails generate migration AddImageableToProducts imageable:references\{polymorphic\}