Building a polymer-solution system

Question:

I want to investigate the behvrior of a polymer in an aquesous solution. I know how to build the polymer, but what is the fastest method to add a certain amount of water to the system and then study its behavior? That is, how can I define the simulation box for the polymer and then add a certain amount of water to the box and ..........

I also need to export this structure to other softwares.

Thank you.