I am my Mother's only named Medical POA. Am I able to add my niece to the existing Medical POA?

No, the POA is for mom to do.  She grants POA for her afffairs, not anyone else.

You cannot add niece Mom has to. Call the lawyer who drew up the original POA and see what needs to be done.

If your mother is sound of mind, she can certainly create another HCPOA document that supersedes the previous document. I would be careful about equal rights - what happens if you disagree on a treatment coarse or where mom should live? I recommend a primary and a secondary where the secondary is authorized to act whenever the primary is unavailable. That way if there's some disagreement, the primary makes the final decision but your niece as secondary could still act as HCPOA when you are unavailable due to distance.