title: How to use recipes package to replace missing values with a constant
body: <p>I can't figure out how to use the recipes package to replace missing numeric variables with a constant. </p>
  
  <p>I did think about using step_lowerimpute, but I don't think I will be able to use it for my case. step_lowerimpute replaces missing values <strong>below</strong> a given threshold with random numbers between <strong>0</strong> and the threshold. In my case that will not work.</p>
  
  <p>For example, I have some lab variable, like lactic acid, which is often missing. I want to replace missing values with an extreme value, such as -9999.  </p>