That's an excellent question, Roger. In hindsight, yes it is called a custom metric. You can ingest any metric with Site24x7 including the business KPI that you mentioned.
Following are the ways in which you can monitor business KPIs
1. If you are using APM Insight, App Parameters is the easiest way to monitor important KPIs in your app.
2. You can monitor the login event via access logs or application logs and push to Site24x7 Applogs. Once ingested you can then form a query and start monitoring them. Alerts can be set based on your thresholds. This will give you KPIs based on log data.
3. If you are into statsD, you can ingest these KPIs via statsD for monitoring