I didn't understand what you meant by combine? RANS, LES and DNS are different approaches and can't be combined. RANS and LES can be picked directly in Fluent. For DNS, there is no specific option. Rather, you can select unsteady, laminar with time and spatial discretization as recommended for DNS (refer to literature). However, this will be extremely computationally expensive and is only recommended for simple geometry, provided you have sufficient computational resources.