Can a 6th-gen Intel CPU handle 32GB of RAM even if it's not on the motherboard QVL?

Yes, your older Intel 6th-gen CPU can almost certainly handle 32GB of RAM — the motherboard QVL is just a list of what they tested, not what works. Intel’s official spec for Skylake desktop CPUs supports up to 64GB, so 32GB is well within range. Just use two 16GB sticks or four 8GB sticks, stick to 1.2V or 1.35V DDR4, and grab a reputable brand like Crucial or G.Skill.