Since there appears to be no default app monitor template in SW for monitoring CRL expiration, using a custom-built solutions may be the way to go.
You can check Thwack Content Exchange for custom app monitor templates built by our generous fellow Thwack community members -> Server & Application Monitor .
Or you can make a custom-built PowerShell script for your own company's CRL expiration monitoring purposes.
You can get ideas from other people in the internet who already attempted to monitor CRL expiration via Powershell, but make sure you follow the guide from SW about How to Create a Windows PowerShell monitor
Here are some link to ideas posted over the internet by people interested in doing the same thing as you.
Just my 2 cents.