Bump.... it's uncomfortably quiet at this forum. Doesn't Arturia have anyone answering questions when there is a lull in the conversation?
I guess it's a tricky proposition; I tried to think of a solution using USB, but as you mention, your ASIO driver will prevent you from pulling from multiple inputs (and you don't have a way to create an aggregate device). There are converters that will convert USB-C to S/PDIF, but that doesn't help you, since you can't 'split' the 8pre USB signal.
From an ADAT perspective, there's an outside chance that using a converter will work, but it's unlikely. You can convert your Kemper output from S/PDIF to ADAT and use that as input into the 8pre, but then the 8pre has to be in ADAT mode and USB recording won't be possible. In the other direction, your Kemper will accept a S/PDIF input, which means that you can convert the 8pre output to S/PDIF and feed it into the Kemper, but I don't think the Kemper will combine it with guitar input and send it out via S/PDIF (even in a reamping configuration).
Sounds like you have a great setup - love Cakewalk and the 8pre is super quiet, but you may have to record your Kemper digitally to an off-board device (I've used an old PC with a coax S/PDIF input in the past) and add it to Cakewalk as an extra step. It may be easier to send the analog output from the Kemper to the 8pre and digitally (to a separate PC/device) simultaneously - then you can just replace the analog track in Cakewalk with the identical digital one when you're happy with the mix.
You've likely seen these, but here's an ADAT to S/PDIF converter:
https://reverb.com/item/1028430-midiman-sam-s-pdif-adat-24-bit-converter-blueThe best solution would be if Arturia said that there's a 'secret' mode that enables S/PDIF on the SYNC BNCs, but that would encroach into AudioFuse territory...I'm going to assume the requisite components to enable that functionality aren't even on the board.
Also, I'm assuming I've understood your requirements - if you were trying to accomplish something else interface- or recording-wise, please let me now (I'm not associated with Arturia, but did work on programming DSP farms for DigiDesign long ago).
Best of luck - please let the forum know if you have any success.