As said in the title, is there a way to add application users in Thorntail WilFly server, much like you would do with \"add-user.sh -a\" script in the full server distribution?
It depends on for what you need the users? Thorntail creates standalone Microservices, which are different to hosted applications in a wildfly-server.
Yes there is, but I have never used it.
https://docs.thorntail.io/2.2.0.Final/#_management
https://docs.thorntail.io/2.2.0.Final/#_management_console
The users you maybe able to create there shouldn't be persistent, because there is no wildfly-server installation as you are used to with a standalone wildfly-server installation, it is all packaged in the jar. A Microservice shouldn't need to be configured after its deployment anymore, at least not like this.
I would recommend to use an external user management via keycloak, which is integrated in thorntail via the keycloak fraction. With the keycloak fraction you can define security constraints to your endpoints similar in a web.xml.
https://docs.thorntail.io/2.2.0.Final/#_keycloak
Another way is to use the security fraction which provides you JAAS support for your microservice.
https://docs.thorntail.io/2.2.0.Final/#_security
The configuration is done via the thorntail specific project-defaults.yml configuration file, where you can configure the fractions via YAML.
A thorntail fraction is similar to a spring boot start dependency with spring, whereby the fraction provides the API for the developement and bundles the implementation and integration into thorntail. The fraction actually is a jboss module which is packaged into the standalone Microservice during re-packaging phase.
See the following links for examples how to use security in thorntail. You should take a look at them.
https://github.com/thorntail/thorntail-examples/tree/master/security
Take a look at the src/main/resources/projects-defaults.yml which contains the configuration for thorntail fractions and the pom.xml which defines the used fractions.