DataObject.AddPastingHandler(this.numericUpDown, OnNumericUpDownPaste);

private void OnNumericUpDownPaste(object sender, DataObjectPastingEventArgs e)
{
	var copiedString = e.DataObject.GetData(typeof(string)) as string;
	if (copiedString != null)
	{
		copiedString = copiedString.Trim();
		double number = 0;
		var success = double.TryParse(copiedString, out number);
		if (success)
		{
			this.numericUpDown.SetCurrentValue(RadNumericUpDown.ValueProperty, number);
			e.CancelCommand();
		}
	}
}