You can create an interrobang with an EQ field (discussed in detail at http://wordfaqs.mvps.org/Overbar.htm). The field code should be
{ EQ \O (?,!) }
When that field is updated, the question mark and exclamation mark will be overprinted in the same space.
When you've created the field and updated it, select it (just the interrobang, not anything before or after it). Go to Options > Proofing > AutoCorrect Options. The interrobang will appear in the With box, and the Formatted text option will be selected. Type
your shortcut (I'll suggest using \ibg) in the Replace box and click the Add button, then OK.
The AutoCorrect will always insert the interrobang in the font and size it had when you created the AutoCorrect entry (an unfortunate side effect of having to use the Formatted option), but you can select it and apply any font or size you need.