This might be what you're looking for, this is a SIP Profile parameter:
<param name="NDLB-sendrecv-in-session" value="true"/>
https://freeswitch.org/confluence/display/FREESWITCH/NDLB

It may also be possible to re-write the SDP per call:
https://freeswitch.org/confluence/display/FREESWITCH/Codec+Negotiation#CodecNegotiation-RewritingSDP

From: FreeSWITCH-users <freeswitch-users-bounces@lists.freeswitch.org> on behalf of Oleg Belousov <obelousov@gmail.com>
Sent: 12 September 2020 22:22
To: FreeSWITCH Users Help <freeswitch-users@lists.freeswitch.org>
Subject: [Freeswitch-users] Add a=sendrecv onto sdp
 
Hi.
Please advise how I can add  a=sendrecv parameter onto the SDP part of 183 Session progress. I'm using preAnswer in lua script to respond 183 with SDP to an invite.
--
obelousov.tel