In-cell validation does not offer the option to both restrict data entry to values in a list and enter any other value.
I suggest that you use a Combobox control.
From the Ribbon select Developer (If it's not visible go to the Office Button, select Excel options at the bottom and select the Popular tab and check the 'Show Developer tab in the Ribbon' box)
In Developer - Controls select Insert and choose the Combobox.
Draw the Combobox on the worksheet
Right-click the Combobox and select 'Properties'
Add a range of cells to the 'ListFillRange' property - this will be the list of values to choose from.
Add a single cell address to the 'LinkedCell' Property - this will be the cell that shows the result - either a selection from the list or another value entered.
You can right-click the Combobox and select 'Format control...' and change how the control is sized and moved. You can, for example, link it's position and size to the cell or cells underneath it.
Once the Combobox is working, to access its properties and formatting, select 'Design mode' from Developer - Controls.