[exim] Address rewriting before subaddressing

Hi,

I have the following problem: I'd like to activate subaddressing but I
have one user who has an alias with a "+" in the mail-address and I
can't give the user the address that would make the address viable with
subaddressing (the user has an alias like "a+b@???" and I cannot
really give him a@???; their mail-address is a-b@???).
Thus I'd like to rewrite the address of the user when it comes in, so
that it lands in the correct mailbox.
Thus:
Mail comes in for a+b@???, it lands in a-b@???
And for all the users I'd like to activate subaddressing as well.
Is this possible? If so how? Is it just a rewrite-rule or do I have to
take care when actions happen?

Thanks,

Niels