% mass properties additional mass
par.mm = ; % mass of the additional mass [kg]
par.mB = ; % horizontal position of the additional mass [m]
par.mC = ; % vertical position of the additional mass [m]

% update mass properties with additional mass
par.I = par.I + ((par.B-par.mB)^2+(par.C-par.mC)^2)*par.m*par.mm/(par.m+par.mm);
par.B = (par.m*par.B+par.mm*par.mB) / (par.m+par.mm);
par.C = (par.m*par.C+par.mm*par.mC) / (par.m+par.mm);
par.m = par.m+par.mm;