Combine PSHELL and PSOLID in one Component

Hi all, 

I am relatively new to HyperMesh and have to mesh a complex structure for a topology optimization for Additiver Layer Manufacturing. 

Unfortunately I can't share a picture.

The structure is one component only and has thin parts with one size very small compared to the others and it has bulky parts. For my understanding shell elements for the thin and solid elements for the bulky parts would be best. 

Is it and how is it possible to split up the structure to assign shell elements and properties to some parts and solid (e.g. tetra)elements and properties to the bulky parts?

Otherwise any recommendations for a general mesh type are welcomed!

For a complex structure maybe a tetra mesh would be best?

Thank you all.