The answer is a little different for SAML based applications, where SiteMinder is the Relying Party.
SiteMinder does not support JIT provisioning by itself. SiteMinder can redirect to an external JIT provisioning service/application to do the user provisioning if a user is not found in the defined User Directories. The JIT provisioning system then responds back to SiteMinder so the user lookup step can be executed again.
The following links provide more information on what is available in terms of Just In Time (JIT) provisioning with SiteMinder.
Dynamic Provisioning of a User Identity at the Relying Party - CA SiteMinder® Federation Standalone - 12.52 SP1 - CA Tec… User Provisioning at the Relying Party - CA Single Sign-On - 12.52 SP1 - CA Technologies Documentation