How to export Reminders task to Todoist?
This is not possible within the App itself.
Although you can download an App called IFTTT, it connects the Apps you use and make them work together.



Sync Reminders

When you connect two Apps in IFTTT you create a Recipe, turns out there's a recipe right for what you're looking for!
You can find one here and here, after you've downloaded the IFTTT App.

Hope this helped you out!