As far as I am aware, it is not possible to generate a drop-down list using a formula rather than data validation, however, the following approach might be utilized to meet your needs.
- A page layout like this:
![enter image description here](https://i.stack.imgur.com/yfPwf.jpg)
-
Data validation formula as following:
=IF($A$2="List",$C$2:$C$8,$D$2:$D$8)
![enter image description here](https://i.stack.imgur.com/r4q42.jpg)
- And also alert option will be disabled ( to be able to add custom data):
![enter image description here](https://i.stack.imgur.com/lfrNU.jpg)
- When you write "List" in the cell "A2" you will get the Column C in the drop down list and if you do not write anything in cell "A2" you will get D column in the drop down list.
![enter image description here](https://i.stack.imgur.com/VwlIK.jpg)
![enter image description here](https://i.stack.imgur.com/X366G.jpg)