DSL customization is inherited when a stereotype is inherited. It is not necessary to modify existing DSL customization when extending the (standard) profile.
For example, to create your own subtype of SysML «Requirement», named, let's say, Performance Requirement, it is enough to define a new stereotype «Performance Requirement» and add a generalization between the SysML «Requirement» and «Performance Requirement».
All predefined symbol properties (style) and semantics rules of SysML «Requirement» will be reused on elements stereotyped by «Performance Requirement».
The subtypes could have their own customization with some basic properties (for example, human name, etc.). Customizations will be merged, and only additions should be defined.
Try to avoid conflicting customizations, as results can be unpredictable.
Generalization between the two customization elements is also possible. In this case, if these rules are not redefined in the subclass (the particular properties are empty), the specific (subclass) customization inherits the customization rules. If both customizations have the same property filled, the rules are not merged; the customization from the subclass will be used.