Currently, this is not possible in Simulink. However, one alternative to consider depending on the use case is to use a custom storage class for the variable and set it to "ImportedExternPointer". A example of this can be found in the documentation here: http://www.mathworks.com/help/ecoder/ug/pointers-for-signals.html?s_tid=srchtitle
Another alternative which is much simpler is to specify Variable-size within a subsystem Inport. Select support for variable-sized signal in the signal attributes and make sure to enter a maximum dimension for the vector and uncheck the interpolate data option. This will generate variable sized assignments.
