It can not be done using ANSYS Meshing in Workbench (WB). With WB Meshing you will be able to import it and then use it for a fluid simulation, but you wont be able to modify the mesh. The option to do this is using Fluent Meshing. In Fluent meshing you will be able to import a .msh file, or even a .cas Fluent file, and work on the mesh by extracting either the volume or surface mesh, remeshing any particular detail or completely regenerating the mesh. Please refer to the documentation and the available training material to learn more about Fluent Meshing.