1. Enter the symbol "X".
2. On the Insert tab, in the Symbols group, click in the Symbol listbox and then select More Symbols...:
3. In the Symbol dialog box:
- Select the Combining Diacritical Marks in the Subset list.
- Choose the symbol that you want to add to "X".
- Click Insert.
Most accented characters, such as those mentioned above, will be found in the Latin Extended-A, Latin Extended-B, and Latin Extended Additional character subsets:
See also this tip in French: Superposition de caractères.