Freeradius proxy: rewriting response messages

[Simone Vendemia]

Hello to all
We're currently implementing a nice setup with Freeradius, using it as
a proxy for a proprietary radius solution.
Our goal is to use freeradius plus freeradius-openvpn-plugin as a
great way to authenticate vpn users; unfortunately we can't dismiss
the proprietary solution (which would be probably the smartest move),
however it's only function is to verify the proxied password.
For the proxy management we've installed dialupadmin on the same
machine, in order to be able to add some accounting information wich
can't be directly done on the proprietary solution.
Till here, no problem at all, the setup works perfectly.
Unfortunately we need to rewrite the response returned to freeradius
by the other server, in order to change the framed-ip-address and
framed-route given by the proprietary radius with the data inserted
previously in our freeradius sql database (mysql) through dialupadmin.
Both the proxy and the proprietary server have the same usernames in
their local db.
Is that a possible setup? We've already searched online without much luck.
Thank you in advance.