I'm sorry, but is Marketo serious?!?!?!
This should be a basic function thats included in even the simplest UI of any platform like Marketo. It's little things like this that make products easy or hard to use, time consuming or easy and quick. Sure it's a lot of work to set it up in the backend for Marketo, but by choosing not too, your transfering labourious tasks such as this to your clients instead.