It is not possible to change the source of initial values for data store memory block. What you can do only is, instead of entering initial value directly in dialog box, you can you use simulink parameter to get the value from base workspace.
Initial values for Integrator block and Memory blocks have diffrent purpose.
It is possible to reset integrator block at any point of time during simulation. In this case its initial value act as output after reset. So it can be external input also.
Initial value of memory block means its output value at time t=0 when simulation is started and nothing is written on memory block. After that output of memory block will change depending on data written on it. Its initial value will never evaluated again at run time. So there is no meaning of providing initial value as external input.
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