Is it acceptable to place an action button on a tab bar?

Consider a two tabbed view, in this case tasks and participants, whereby in the task view you are able to view cards of the tasks. Within these Task cards you are able to edit the content and assign them to participants.

It would also be required to create new tasks and this is where the question comes: Is it acceptable to have the 'create new task' button on the 'Tasks' navigation tab?

Here is the visual where the navy blue button on the tab bar will add a new task:

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I'm reluctant to use the floating action button (FAB) because:

  1. adding a task is not necessarily the most common action in this view and
  2. the FAB is not so commonly used on iOS, nor is it in the iOS guidelines.

This design is for both Android and iOS mobile platforms so the answer should take into consideration both UI practices.

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