I've just found the solution myself.

In forum1 I created new categories exactly as in forum2 then move all forums in forum2 to their categories in forum1. The "move" option can be found in the "Modify the forum" window with the button "Move this forum into the other forum". Finally I un-install forum2. That's all!

How great the D3 world is! It's something beyond the OOP (Object Oriented Programming) philosophy.