This should be possible.
In your Email configuration under the "Receiving emails", enter the IP address of your MTA.Then in the "Sending Emails" section, add the domain names and associated relay, which in this case would be your other antispam tool.
In the Email Policy section, simply disable the engines you do not want to use and configure the one you would like to use.
As for the headers, it will add "received from" headers by default. If you want to remove them, there are workarounds. I suggest you read this post :
Hope this answers your question