Simulating a DC motor with a constant friction torque
Your block Kt/(R+Ls) is torque. V/(R+Ls) is your current. Kt is torque per amp (Nm/A). You simply add a value after Kt/(R+Ls), if it's constant. Unfortunately, many loads are speed or position dependent and then it get's much more complicated (math wise). Model-wise it's pretty easy.