Are you meaning when the number of Active threads reaches a certain threshold or are you looking at an alert at the rate of increase?
The former is easily achieved by creating a Metric Event (Setting > Anomaly detection > Metric Events)
Then using the Metric builtin:tech.websphere.threadPoolModule.ActiveCount and applying any filters for the scope e.g. ProcessGroups and Pools you can set a defined Threshold limit to alert on.
With regards to the latter. I don't know how to alert for an increase or hike