How do I move existing tasks form Outlook to todoist?
If you're referring to tasks added through Outlook's native "Tasks" feature then unfortunately, that's not possible. There is no interaction between Todoist and Outlook's Tasks.
We may consider adding such option in the future, though and you're welcome to vote for this request in our Votebox: http://todoist.com/Vote/showProposal/194/
I found a easy way by dragging a task from an Outlook task list into the Todoist box where you normally type the task. Then erase the first line (Outlook Task Headers). Not elegant but it works.