In Zendesk, the option to export account data is available only from the Professional and Enterprise Plans. When the helpcenter is activated in Zendesk, the export file will no longer contain the forums and solutions. For more information on this, please refer to this article.
The Solution and Forum data could be derived using API, and you could share that with us separately to be imported into your account. For more information on the API, please refer to this documentation.