[exim] how to change a recipient in a transport?

Hello,

I have an accept router, that uses a smtp-transport to deliver the
message. But before the delivery the recipient has to be changed (a
certain string has to be prefixed). The new recipient is given in a file
and lsearch would return the right thing. But how can I tell exim to use
what lsearch returns? Right now I only have a condition which checks if
the account exists or not.

Regards
Marten