Unfortunately not. That is - you can't sync their flag itself or make all flagged emails be added to Todoist, but you can use the regular "Add email as task" option with any email which will add a task with a link to this email.
I feel the flag functionality is really great in outlook, as it is quite snappy to take note of or closeout on any action.
I was using Todoist as my primary todo list, but the number of clicks I had to go through to add an email was a big deterrent .
The biggest failure I faced was when I read emails on phone, and wish to have it on my action list, I need to wait to log onto outlook to sync with Todoist or do a manual typing onto the todoist mobile/web app.
As of today, I am phasing out my usage of todoist for managing work items, and using it for only personal. :(
Please explore the option of flagging it. :'-(
I second Tanuj