I got the game working with my PS2 to USB converter with X360CE. While not the same gamepad, they are both directinput devices, so this should also work for you:
1. Download the 64bit version and place the x360ce_x64 application inside the same folder as the game launcher -- "EoCApp.exe" (should be located in steamapps/common/Divinity Original Sin Enhanced Edition/Shipping).
2. Launch x360ce_x64 from inside that folder and create the profile as you normally would.
3. Select the "Game Settings" tab in x360CE. Select "x360ce_x64.exe", check all 64-bit checkboxes under the Xinput Files on the right. Push "Apply/Synchronize Settings". Do the same with EoCApp.exe -- if they are already checked, great! (If you get any popups, just click continue.)
Save, close X360CE and then launch D:OS EE.
I hope this works for you, as playing this with a controller is pretty darn slick.