If you Format -- Cells -- Number -- Category = Custom, then choose eg:
$#,##0.00;-$#,##0.00 and change the $s to ?s (that's "euros", in case it
doesn't come out) using ALT+0128, will it still work maths-wise? Seems to
here, with the add-in disabled.
Either way, NB that the symbol won't display in some fonts eg: Courier.