Here are two potential options to provide additional funding for students.
The trainee may be able to receive stipend supplementation if provided from non-sponsored project funds. No service to the university for the stipend supplementation is required.
Wages over and above the stipend could only be charged to a sponsored project if there is work effort on a sponsored project separate from the one funding the stipend. This is not allowed by all sponsors. Check sponsor requirements or contact your grant accountant for more information. If additional effort would cause the student to exceed standard graduate student full-time status, approval from the Graduate School will be needed.