Yes.  This is a good idea since the GSU health plan pays a percentage of your costs rather than 100%.  By combining them, you are maximizing your benefits.  If you were to file a claim with your GSU health plan, you can, at the same time, file the remainder with your HCSA.  Simply fill out the GSU health insurance paper and attach the original receipt, then attach a photocopy of that original receipt to your HCSA claim form identifying that you would like to claim the remainder of your original claim and then mail them both to the appropriate address at the bottom of the Greenshield forms.  There are separate addresses for different kinds of claims (for example, eye glasses prescription would be sent to the vision care address).

If you are claiming non-dental health expenses you can put the GSU form and the CUPE HCSA form in the same envelope (remembering to use the GSU ID number on the GSU form and the CUPE ID number on the CUPE form), and Greenshield should be able to process the primary claim on the GSU plan and the remainder on your HCSA.