If you want to format the HDD to create a new partition you will definitely lose your current data.
I found this on Reddit, (which is not completely on topic but it fits your case imho):
I just did this. It shows the exFAT partition but makes you format the drive, then recombines the partitions and formats them as one. I lost my stuff that was on the other partition. It wasn't a lot or anything too important that I care about. But yeah, it doesn't look like it supports partitions.
By the way, there aren't many advantages in storing PS5 games on an external storage, because you can't play them from it. You always have to copy the game on the internal storage in order to start it.