Transformation may be possible. Whether the plasmid replicates depends on whether the pET21 origin of replication can function in S. mutants. It is not common for Gram negative origins to work in Gram positives. If that is the case, you can transform the plasmid, but it will integrate into the genome if sufficient homology exists and a homologous recombination pathway is sufficiently active. What is the purpose of the transformation? You may be able to get around the problem by cloning a S. mutant origin of replication into the pET plasmid.

It will be valuable to add the ORi of S. mutant into the pET plasmid , it will be called Shuttle vector which can replicate in both negative and positive gram bacteria.