I've already done that using drop down menu (Special) in the Interaction Module, which is same as what you described. When I run gravity load for the rest of the structure , the added point mass in not included in the analysis (only the mass of other deformable elements), I checked that by summing RF2 at supports. Ive tried all the procedures I found from different forums, but none of them counted for the added mass.
i have used rigid bodies and assigned point mass at them in many simulations and specifically at impact problems. I am sure that you are missing something. First to get Reaction force you have to apply boundary conditions in your rigid body. You can also check kinetic energy to see if the mass affects your analysis.
In the case you couldn't add gravity force acting on your part due to any reason I think you can simply add another force with the same direction and value of W at a reference point coinciding the center of the mass of the mentioned part.
Thanks for your inputs. Actually I applied BC at the RP to be connected to the rest of model. Lets say the case of three point bending test, for the loading beam I modelled it as rigid body and applied the BC at the RP except the vertical direction. I can apply the mass as Point or patch load as mentioned by Seyyed in above comment. I made a quick model for three point bending test (see attached), the loading beam (rod in the model) is rigid body with applied BC at RP. Other aspects of model are working fine (contact, material models, etc.). I just want to consider the mass of loading beam prior to start loading the beam to failure. Although I can do other approaches to do so, I am interested to have it as mass which is the case in the experiment.