Can I edit the xsd schema file in JBoss EAP 7?

Resolution

Although not recommended, one could edit the schema file in JBoss EAP 7/Data Grid 7/RHSSO 7 adding or removing attributes.
However the change will be only minimal and wouldn't play much a difference since the subsystem underneath does not use the new attribute and it won't recognize it.
Example:
Go to $JBOSS_HOME/docs/schema/jboss-as-infinispan_12_0.xsd used to validate the infinispan subsystem. Let's add an random attribute banana below the complex type state-transfer called example:

        <xs:attribute name="example" type="xs:long" default="240000">
            <xs:annotation>
                <xs:documentation>
                    example
                </xs:documentation>
            </xs:annotation>
        </xs:attribute>

1. Adding the property example in state-transfer element in the standalone-ha.xml for testing:

               <distributed-cache name="offlineSessions" owners="1">
                   <expiration lifespan="900000000000000000"/>
                   <state-transfer example="0"/>
               </distributed-cache>
   

2. Let's start JBoss EAP with this property:

   OPVDX001: Validation error in standalone-ha.xml 
   |
   |  373: <distributed-cache name="offlineSessions" owners="1">
   |  374:     <expiration lifespan="900000000000000000"/>
   |  375:     <state-transfer example="0"/>
   |                           ^^^^ 'example' isn't an allowed attribute for the 'state-transfer' element
   |                                
   |                                Attributes allowed here are: example, chunk-size, timeout 
   |
   |  376: </distributed-cache> 
   |  377: <distributed-cache name="clientSessions" owners="1">
   |  378:     <expiration lifespan="900000000000000000"/>
   |
   | 'example' is allowed on elements: 
   | - server > profile > {urn:jboss:domain:infinispan:12.0}subsystem > cache-container > replicated-cache > state-transfer
   | - server > profile > {urn:jboss:domain:infinispan:12.0}subsystem > cache-container > distributed-cache > state-transfer
   | - server > profile > {urn:jboss:domain:infinispan:12.0}subsystem > cache-container > scattered-cache > state-transfer
   |
   |
   | The primary underlying error message was:
   | > ParseError at [row,col]:[375,21]
   | > Message: WFLYCTL0197: Unexpected attribute 'banana' encountered
   

So it is pointless changing the definition.
Also not the validation cannot be skipped and who is responsible the validation is ApplicationClientConfigurationPersister.java, via the method load, which has a filepath.