You'll have to ask MSI about that!
I know what Asus told me over the years - many things were just wrong although they should have known better.
You cannot always rely on what manufacturers, customer support or hotlines say when it comes to mainboards. It's good that you can rely on Scythe's answers to 99%.
I can only repeat - as far as i am informed there are no adapters on the market that could generate the signel for the fourth pin (speed control) from the 3 pins of a normal fan. Ask MSI to send you a link to such a product, please!
All other things work: 4-pin fan on 3-pin header (w/o regulation or with voltage regulation based on 3 pins), 4-pin molex to 4-pin 5,25" power cable etc.
Bye Oli