Transparency is handled by the material that's applied to your objects.
I create duplicate materials (magenta _100, magenta_65, etc) where the number prefix is the opacity of the material. I then use the "set material" action to add these changes.
You can also try the "set material color" action which will give you control over the material's opacity as well - just keep ind mind this will affect all objects with that material assigned to it, which may or may not be what you need