SMS - No Way To Upload List?

Just subscribed to SMS only to find out you can't upload a list or text your current users. It looks like the only way to text people is for them to manually go and opt in to text messaging - is this correct? If so, this is a HUGE disappointment, because we already have a list of 6000 phone numbers from supporters that have opted in via our website. Other texting platforms allow this, too, so I really hope this is something CC changes soon.