Students cannot use Turnitin or its plagiarism checking functionality unless an assignment has been set up by a lecturer in a Myplace a class page.
Staff could make submissions for the sole purpose of plagiarism checking via a 'test' or other (non-registry) class, by simply adding a Turnitin assignment when editing the page (Go to Myplace page, 'Turn editing on' then select 'Turnitin assignment' from the 'Add an activity...' drop down list) The member of staff can then 'submit' an assignment and view the originality report. (This assignment could be visible to all students in the class, or hidden from view using the 'eye' icon when in editing mode)
If you need to submit on behalf of someone else, then you can manually add that user to the class page as a student (see *Staff* Enrolments - Adding Informal Roles to a Class), and then make the submission on his/her behalf (see *Staff* Turnitin Assignments - How to Submit for your students). Note that work belonging to others should not be submitted under your own name nor be submitted without their knowledge, and a submission receipt will be sent to a user's email address when you make a submission on his/her behalf.