I wonder if it is possible (OOTB) in a combobox column, when other option selected, a textbox is displayed and user enters custom text.
You can configure the column to allow fill in options.
A way I've done this is the past is to create a list to store my column values. Then in my desired list, create a lookup column to that list and set it to allow fill ins. Then with a workflow on item creation or change, if a user enters a fill in option, I create that entry in my list and then update the item to that newly created value.