In documentation is documented only one provider OpenSSL.
https://docs.wildfly.org/25/WildFly_Elytron_Security.html#OpenSSL

In default WildFly configuration are 4 SSL providers which can be used. Default JDK SSL engine which doesn't require any configuration. Two provider are configured as standalone providers openSSL and Elytron. And one aggregated provider which aggregate OpenSSL and Elytron

OpenSSL provider can used as is described in documentation. 

Elytron require additional configuration which is not described in documentation. Some  information about Elytron provider can found in comments. https://issues.redhat.com/browse/WFLY-15427

It would be great to have described benefits of aggregated providers. Why are aggregated into one provider and how this aggregation work. 

In section 14.4 https://docs.wildfly.org/25/WildFly_Elytron_Security.html#adding-additional-native-libraries
Is described manual update of OpenSSL library in WildFly. And it look like this mechanism should be possible to import any implementation of SSL and create new SSL provider in WildFly with custom implementation. Of course WildFly have some specific requirement on new SSL provider which should be described.