Converting meshes to 3DSolids is possible, but not the legacy Polygon or Polyface meshes. They have to be the new MESH entities.
To convert legacy Polygon or Polyface Meshes into the new MESH (SubDMesh) objects we can use the _MESHOPTIONS command. Once the objects are converted into the new MESH class, in case they are watertight (that is, they enclose a volume with no holes) they an be converted into 3DSolids using the _CONVTOSOLID command. This is shown in the following video.

In case they are not "watertight" they could be converted into solids using the _THICKEN command. Other option would be to convert the meshes into Surfaces and using these surfaces to SLICE a 3DSolid.