Override Expires header for proxied content in Apache

I am serving proxied content with Apache that already contains an Expires header. Using mod_expires' ExpireDefault to override the already present Expires header does not work, the old header is left untouched.

Is there another way to override the Expires header? I tried using mod_headers, but it seems I cannot do date calculation there. I also must override the Expires header, I cannot use Cache-Control with max-age for this.

Regards,
Jochen