You defintiely can connect yourself to your grandparents, but without trusted status you won't be able to make changes to their profiles. To add them as the parents of your parents, you would go to your mother or father's profile. Click on the edit tab, and on the right hand side, there is an option to add their father or mother. Click there, and at the top of that page it says "If you know that this mother already has a WikiTree profile enter their WikiTree ID here:" Enter your grandparent's ID, and it links them to your parent.
I hope this helps! If you need any clarification, let me know.
