As far as I understand you are looking for a way in which you can specify the initial value for a signal in a Simulink model.
Probably the IC block will fit your needs.
The IC block sets the initial condition of the signal at its input port, for example, the value of the signal at the simulation start time (t_start). The block does this by outputting the specified initial condition when you start the simulation, regardless of the actual value of the input signal. Thereafter, the block outputs the actual value of the input signal.
Hope this helps!