Yes, include them using the Binary element.
<Binary Id='MyCustomActionBinary'
SourceFile='$(var.CustomActionProject.TargetPath)' />
This will make them available to your CustomAction where you can use the BinaryKey attribute to reference the Binary:
<CustomAction Id='MyCustomAction'
BinaryKey='MyCustomActionBinary'
DllEntry='MyCustomFunction'
Execute='deferred' />