If you would like information to be searchable in JIRA service desk that is in another wiki you will have to migrate that into Confluence. there might also be a plugin. You can learn more about migrating content here: https://confluence.atlassian.com/conf51/importing-content-into-confluence-336169436.html
If it is already in a Confluence Space you could move it to your service desk knowledge base space in Confluence or just point service desk to that space as your knowledge base.
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