Hi,
You would have to modify the code in the file plugins/hikashop/datepickerfield/datepickerfield_class.php
There, you would need to modify each place (4 places) you have the code:
At each place, you'll have to check if you're in the backend or not.
It can be done with the hikashop_isClient('administrator') function call.
So with a bit of programming it's possible. However, note that you'll loose your changes when you update HikaShop.
Thus, the best would be to make a copy of that plugin and rename it differently (similarly the plugin code needed to be updated here and there) so that you would be safe in the long run with that change.