1. Put =CHAR(96+ROW()) in cell A1.
2. Drag down till A26 to generate a to z.
3. Put =CHAR(64+ROW()) in cell B1.
4. Drag down till B26 to generate A to Z.
5. File > Options > Advanced > Go down till you find Edit Custom Lists (You have to go down till bottom) and click this
6. Now, you can import your range as below.
7. Now, any time you need this sequence just put a or A and drag down. The sequence will be generated.