Provided you know your initial conditions (ICs) in relative coordinates, you can either determine the ICs in absolute coordinates or simply measure the absolute variables of interest during simulation, depending on your desired application(s).
If you setup the model using your relative ICs and force all joints (and other DOFs) to be static, you can use absolute sensing components to directly evaluate the absolute ICs. This will give you calculated absolute ICs based on the known relative ICs, which you can use to replace the relative ICs if desired.
If the goal is to simulate in absolute coordinates, without needing to know the absolute ICs directly, you can simply use your relative ICs to setup the model. Then you can easily control and measure absolute variables and motions directly using the appropriate components, without ever needing to know the ICs in absolute coordinates.
Brandon J DeHart