Ankush Pratap Singh, you can do this by using the following steps:

1. Generate random points within the geometry. Rhino is an efficient tool to perform this step.

2. Assign random pore size to each point.

3. Generate desired pore shape at each point with assigned pore size.

4. Subtract the pore shapes from the geometry.

To generate a geometry with a desired porosity, you have to play with the range of assigned pore sizes.

Thanks Masud Da