I think you may need to add a test point at this point to enable the controllability of this point . There must be some way to do that automatically, I don't know how exactly.
Anyway, if you want to add it manually in the RTL code then you need to add a dummy flop and a mux controlled by the test_mode pin . in test mode the mux selects the output of the dummy flop and discards the external pin (shift enable) , while in functional mode the mux selects the external input .