at least in 2.6.3 there is no support for it in the portal core.
The last thing I recognized is the discussion of this topic by the portal team:
Nevertheless, because we needed that too prior to that discussion, I implemented at least a minimal API (just for our needs, i.e. database is supported only) based on portal 2.4.1:
Last december we have managed to upgrade to portal 2.6.3, in which the iidentity design has been majorly changed.
If there is a demand for that, I can provide the necessary changes in jira too.