Yes, multiple ArcGIS Online subscriptions for an organization can be merged. If an organization has an unused subscription, the number of users and credits within the subscription can be added to an active subscription. The merging of subscriptions is done by Esri Customer Service.
Note: Content within these ArcGIS Online accounts cannot be merged. Users cannot combine ArcGIS Online entitlements from separate customer numbers.
Content within one ArcGIS Online subscription can be transferred to another subscription by the ArcGIS Online administrator using one of the following methods:
Note: ArcGIS Online Assistant and GEO Jobe Admin Tools for ArcGIS Online do not fall within the scope of Esri Technical Support. To transfer content via ArcGIS Online Assistant, refer to FAQ: Is it possible to transfer data from Portal for ArcGIS to ArcGIS Online?