Currently, it is not possible to add "Watchers" as an option when configuring a Security level, and this could come in handy for a few scenarios like:

• Users who were already watching the issue can lose access to it when a security level is set on the issue.
• We may want to create an issue with a security level from the get go and add watchers from an automation or script using the REST API Add watcher.

A way to achieve a similar result is to employ a custom user picker field, and to add the users there as a kind of "watcher" replacement, as custom user picker fields can be added to a security level, but this has it's own set of requirements and configuration work to be done, and it would be much easier to use an existing system function.