Can I upgrade my Apple Watch Series 7 to add cellular?

no, it has different hardware than the GPS model, best you can do is to sell it and purchase the one with cellular support.

Here is the Apple Return Policy

https://www.apple.com/shop/help/returns_refund

When did you purchase your watch? It could still be within the return period. If it is still within the return period, you can return it for full refund and buy the cellular version.

Can I return Apple Watch 7 GPS and exchange for GPS + Cellular online?