Hi!
The answer is: You don't (add weight)!
When you build a plane you must always have in mind the removal of unnecessary weight! That's paramount!
The Dremel sanding drum and a carbide router is your best friend! There is always things that could be removed or be improved on in any airplane kit once you got the "feel for it"! Never settle for: "Just build it according to the instructions adding part A to part B"!
Ask yourself : Is this piece of wood/ really necessary?

Things that could be removed or improved on are: too much wood, too heavy wood structures, wrong equipment, lighter tail-wheels, move servos, tank and all other equipment forward etc.

I have never in my 32 years in this hobby fallen for the temptation of adding lead to any model, be it scale, racing or sport
It's an easy trap to fall into...But if you want a good flying model ...work at it!