Fill hollow models for better 3d prints

[Picko]

Greetings, I have been using mpfb for some time now, creating all sorts of different humans in order to 3d print them in a really small scale and add them to a maquette. The problem is that the humans are hollow on the inside and this is a problem for the printing procedure. Is there an easy way to fill the human?

PS. I know about blender's fill action with Alt+F but the problem is that there is no good way in order to completely fill the human with this action.
Thank you in advance.