Hi Mark,
this CFX Concert Grand Pack available in various plug-in formats and as standalone version for PC and Mac. There is no YEM compatible version.
In theory, one can re-sample these sounds (e.g. using a program like "Sample Robot") and make new, YEM compatible Voices from those samples.
But there is a "little" problem: The PSR-S975 has only 768 MB of Expansion Wave Memory (Genos has 3 GB). But this CFX plug-in Pack needs 50 GB of Memory for a full installation or 12 GB for "compact" installation. On a computer the sound generation is realized by "Disk Streaming", but on our keyboards, all the samples must be completely loaded into the Expansion Memory at Pack Installation. And as you can see, this is not possible in this case.
Of course, an experienced sound programmer could make a relatively small Pack (that could be used on the S975) from this CFX plug-in Pack. But then the samples had to be looped, and only a few samples of the original Pack could be used. So the final quality (played on e.g. an S975) had little to do with the original CFX plug-in Pack.
P.S.
I recommend you to try the new, free "New Year Pack", programmed by our forum member Francesco Massa. This Pack (.ppf) contains various high quality Steinway D-274 Grand Piano Voices:
>>>
https://www.psrtutorial.com/forum/index.php/topic,57971.msg449447.html#msg449447P.P.S.
Some of the additional Voices of this Pack are Genos exclusives and cannot be installed on other models. But you can just ignore these Voices when using this Pack with the S975.
Best regards,
Chris