The board has 4 DIMM slots, so you *could* try adding a further 2x4GB kit, as close to your existing kit as possible (speed, timings, latency) - for a total of 16GB, as see how that works out. BUT, there's no guarantee they'd 'play nice' with your existing kit.
Personally, I'd just opt to replace with a 2x8GB kit.
PCPartPicker part list /
Price breakdown by merchant
Memory: G.Skill - FORTIS 16GB (2 x 8GB) DDR4-2400 Memory ($142.87 @ Newegg Marketplace)
Total: $142.87
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-11-01 10:10 EDT-0400
If you're looking at 32GB, that's about another $150 wasted on a gaming setup. You're just not going to utilize it.
PCPartPicker part list /
Price breakdown by merchant
Memory: Corsair - Vengeance LPX 32GB (2 x 16GB) DDR4-2400 Memory ($299.66 @ Newegg Marketplace)
Total: $299.66
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-11-01 10:12 EDT-0400