Yes. You put it in your Normal worksheet template (or use a different name for your template).
You can find it in the Mathcad Template folder; it will be normal.mct for M11 and below, and nomal.xmct for M12 and above. Simply open the template, copy the function to it, and save the template. All future worksheets created with this template will include the function.
If you want to make its appearance less obtrusive, put it into an Area (main menu, Insert-Area) at the top of your worksheet and collapse the Area (see Help if you're uncertain how to do this).
Have a look at the worksheet I posted in the WRITEPRN thread for examples of collapsed Areas containing functions - the four Areas are all in my normal template.