For now it's not possible in Regnum. But, you could use a DirectX utility called "dxdiag".
Disclaimer: Please do this knowing in advance that your monitor supports the rate specified and under your own risk.
Instructions on how to do it:
- Start menu -> Run "dxdiag"
- Go to "More help" tab
- Click on the button "Override" after the "Click this button to override DirectDraw's refresh rate (for advanced users only)".
- Tick "Override value" and put the desired refresh rate