The functions palette has a choice called select VI...(icon looks like a VI icon inside a dialog "bubble") choose that and browse to the VI you want to insert as a sub-VI.
If you use a function frequently you can put it in the user.lib directory and it will automatically show in the user lib subpalette of the functions palette. If you want it to be a separate button somewhere else edit the menu by selecting Edit Palette Views from the Tools->Advanced menu.
