Can I add a new sequence after the campaign has started?

You can add sequences to leads whose entire sequence step has finished and they'll be re-activated to receive the new sequence step.

Example, you have a 2 step sequence

Monday & Tuesday.

You have uploaded 200 leads to a campaign.

By Tuesday 150 leads have received all the steps in the sequence (1 & 2) and are "completed".

However on Wednesday you want to add 1 more sequence step on Friday.

When you do that, the 150 leads which were "completed" will take priority to send messages to first in the sequence for Friday followed by leads that are yet to be reached out to first or leads in step 1!