Just to provide some update on this ticket. Currently JIRA Service Desk supports all custom fields except Group pickers, Project pickers and Version pickers. The Project one is not relevant to the portal's users – please let us know how you'd use it if it's relevant to you so that we can understand your use case. We haven't heard many customers asking for groups and versions and therefore they are not on our current roadmap. We'd like your use case for these ones, esp. the group field.
In terms of add-on fields from our partners, e.g. nFeed, this is something we would like to have in JSD in the future but are currently not on our short to medium term roadmap.
— JIRA Service Desk team