Yes, but you'd need to know the character codes of the characters to be replaced. If you know that, you could use a wildcard Find/Replace, where:
Find = [^0191^0233]
Replace = ^&
where the numbers following the ^ characters are the 4-digit character codes (for ¿ and ß in the above example) and you specify both the Find and Replace fonts.

If the result of doing this is that you'll have words etc. using a mix of fonts, this is not a good solution (IMHO); you'd do better to use a font that does support all of the characters in use.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]