Solid mesh to fluid mesh

I'm currently trying to generate a pure Hexamesh for a CFD project. Is it possible to first generate a solid mesh and then convert it to a fluid mesh? I've tried 'CFD-Tetramesh' but it seems like there is no option to make the core hexa instead of tetra. 

P/S: I'm working on an air flow in a straight pipe. 

Any help would be appreciated.