I have an SL16R which - as you say - burns one analog input channel for talkback. However, the talkback channel input source can be set to Network. If you go to the AVB section of your Digital Patching page (in Settings) you can then nominate an AVB channel for your Talkback channel to reveive from.
On your SL32 mixer you can set its Talkback channel to send on the same AVB channel and hopefully that will then make the connection you need. That is, when you hit the Talk button on your mixer end the signal should go through to the talkback channel of your SL32R III.
Probably best to keep Talk always engaged on your SL32R III so that it's open when you press Talk on the mixer end.
Hope that helps.