Let's say you had, in cell A1, a very simple formula, =A2+A3.
You can then apply Data Validation to that cell, to restrict user input, for example to a list.
As long as you apply them in that sequence (formula first, then DV), then Excel will let you do it.
The formula will calculate correctly, even if the result is not an allowable input as far as DV is concerned.
You will then be able to overwrite the formula, using DV.
Try it.

But I would be careful with this.
Partly because, once you've applied DV, it will be tricky to input the formula a second time, or even to change the formula, if you want to do that.
You'd have to take off DV to do that.

I would consider doing this in separate cells - have a formula that gives you one result.
Then have a separate DV cell, for optional user input.
Then have another formula, which says, IF the DV cell is blank, pickup the result of the first formula.
If the DV cell is NOT blank, pick up the value in the DV cell.

I hope I was able to give you some insights.

You can get more help with https://www.o365cloudexperts.com if needed.

Regards,
Jerry