If you do it that way, your letter will be much smaller than the
surrounding text. You can use the Overstrike field as macropod shows,
but then raise the period on the second panel of Format Font, or if
that isn't precise enough, by nesting another field that moves a
character up/down without changing its size; if the dot should be
bigger than a period (I think it should?), then use the centered dot
character rather than the period character.
If you wanted to put a dot over a letter (as in Polish or Latvian
[IIRC]) and don't have a font with Polish or Latvian characters, you
can go to the "combining diacritics" part of a well-endowed font like
Tahoma and find the correct diacritic. (Once it's in your text, you
can try changing it to the font you're actually using. If the
character doesn't happen to be there, it'll stay Tahoma.)