Using the “Combining Enclosing Circle” character in Word
One trick to do this is to adjust the tracking. In Word (2007), you can highlight the two adjacent glyphs, go into the Font dialog, choose the character spacing tab, set "spacing" to "condensed" and "by" field to something such as 10pt. Check the preview for alignment.
This essentially sets the spacing between the two glyphs to a negative amount, causing the second one to overlap the first one. If you change the typeface or size, you will need to tweak it.
This appears to be a font problem. Testing with web browsers produces similar problems as attempts at using U+20DD in Word. Typically, if a font contains it, it is not really implemented as a combining mark in the font but more or less as a spacing character – instead of “?” in a circle, you get just a “?” followed by a circle.
Font support to U+20DD is limited but contains some fonts where it is implemented as a combining character, at least Code2000, GNU FreeFont fonts (e.g., FreeSans), STIX fonts, Symbola, and XITS. In some other fonts, such as Cambria, there has been an effort on making U+20DD combining, but it has failed (the circle is badly misplaced relative to the base character).
Unfortunately, in the fonts tested, the circle mostly tends to be too small for “?”, but the technique would work for smaller characters, though the results really vary by font and by character. Below is an encircled “?” and an encircled “à” in the FreeSerif font (of the GNU FreeFonts set):
