Hi Krystian, are you referring to the "extension" property in the data model? If so, then that is currently supported by FHIR repository in HealthShare. Resources can be created in the repository with extensions and these will be saved. I'm attaching an example of a patient resource that contains multiple extensions. Note that this is a DSTU2 resource, which is the FHIR version that HealthShare currently supports. The current version of FHIR is STU3. HealthShare will support this in a future release.
EDIT: I *thought* I could attach a file, but that seems not to be the case. In any event, you can download the FHIR DSTU2 examples directly from the hl7.org site as XML or JSON:
And you can look through those for examples of extensions.