Another option exists. Make a named range that expands dynamically. The data validation list is then defined using the range.
Insert the following into the named range box and give it a name to create your dynamically expanding range:
=OFFSET($A$1,0,0,COUNTA($A:$A),1)
Replace $A$1 with the highest cell in your range. Replace $A$A with the column(s) that the range is in.
OFFSET directs the specified range to a set of cells. The height of the range is determined by COUNTA(), which is in the fourth place of the OFFSET formula. The quantity of non-blank cells is counted. In turn, this causes the fourth value of the OFFSET formula to rise when you add a value, giving you a widening range.