I doubt she could, since the hammer was forged by Odin force!! I will say Thor but sersi is one of the most powerful heroes/ females on earth she will give him hell!
AFAIK, Thor's hammer protects him from transmutation. That limits Sersi to conventional forms of super-combat (energy-blasts, punch/kick) plus some psychic tricks like illusions.
Who has ever managed to manipulate the hammer? Surfer's done some impressive stuff to Beta Ray's hammer and managed to turn Mjolnir from its cane form. Sersi's manip. powers are comparable to his. But in both cases, the hammers were not in possession of their owners. I doubt she could do anything to Mjolnir in Thor's hand without further evidence to consider.
If, as the OP suggests Thor stands there and doesn't use his hammer to block her, I think she can turn him into a frog. The hammer...well I'm not sure she can transmute that. No evidence either way.