No, it is not possible for an account administrator to reserve credits for any member of an ArcGIS Online organizational account. This is a known limitation.
However, account administrators are given full access and privileges to an ArcGIS Online organizational account. These privileges include:
• Managing an organization's financial resources.
• Managing users in the organization.
• Creating custom roles.
Instead of reserving credits for a member, administrators can set limits or completely remove a user’s ability to consume credits via assigning custom roles for selected members.