Somewhat related, if you have a font with Holy Water at home, and it's nearly empty, can you add more water (unblessed) to it and retain its sacramental status?
No.
I have heard that Holy Water can be topped up with normal water as long as the Holy Water makes up more than half. But I guess, if the font is "nearly empty" it's too late, as the normal water would be more than the Holy Water.