Students cannot use Turnitin or its plagiarism checking functionality unless an assignment has been set up by a lecturer in a Myplace 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 'Assignment' from the 'Add an activity or resource' link - the assignment settings are defaulted to have Turnitin enabled, but the originality report not available to students - all of the assignment settings can be changed according to requirement) 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)
You cannot submit on behalf of someone else, as each Turnitin user must accept Turnitin's End User Agreement, and must also tick the University of Strathclyde submission statement to agree that the submission is all their own work and may be subject to plagiarism checking.
Staff can have a test or other non-registry Myplace class set up at any time by request by contacting the helpdesk