PLY is a generic file format that can store any mesh entities with information attached to it. It is mostly used to represent surfacic meshes, and this is what most softwares that use this file format will expect (including Tetgen), but in principle it can be extended to store tetrahedral meshes.
If you want to store tetrahedral meshes in PLY files and use Tetgen to manipulate them, you will need to modify Tetgen or use it as a library (Tetgen is implemented in C++).
An example of using tetgen as a library is the GEOGRAM library that I'm developping:
For reading and storing PLY files, I am using the following library:
If you do not plan to modify Tetgen / use it as a library, another possibility is to write a file converter that converts between 'tetrahedral mesh stored in PLY file' and native tetrahedral meshes formats supported by tetgen.