FIV or VIV on a deformable structure will require a two-way couple FSI simulation setup. If the structure is rigid and the deformation is not a crucial aspect of the investigation, you can simplify the problem by using the "Six-DOF" option in the "Dynamic Mesh" tab of Ansys Fluent. Fluent also offers the "Overset Mesh" feature which will help you set the simulation. The mass and moment of inertial properties can be defined inside the Six DOF properties tab. Alternatively, they can also be defined by an UDF. If the motion is restricted to a particular direction and if there is no rotation of the body, you can constraint the 6-DOF using the properties panel or an UDF. See the following tutorial on setting up an “One DOF Translation” using the 6 DOF properties panel:
ANSYS Fluent: Using the Six Degrees of Freedom (Six DOF) Solver
You might also need the "Implicit Update" option in the Dynamic Mesh tab to help achieve convergence.
General advice: Start with a 2D case before moving on to the 3D case. Consider a rigid body motion before proceeding with the Six-DOF model.