Here are two methods to add site collection administrator to OneDrive for Business site collection.
1.Go to SharePoint Admin Center -> More features -> User profiles -> Select the user profile you want -> Manage site collection owners -> Click browser -> Select the people you want.
2.You could use following PowerShell to achieve this.
$AdminSiteURL="https://tenant-admin.sharepoint.com"
$OneDriveSiteUrl="https://tenant-my.sharepoint.com/personal/emily_tenant_com"
$SiteCollAdmin="******@tenant.com"
#Get Credentials to connect to SharePoint Admin Center
$Cred = Get-Credential
#Connect to SharePoint Online Admin Center
Connect-SPOService -Url $AdminSiteURL -credential $Cred
#Add Site Collection Admin to the OneDrive
Set-SPOUser -Site $OneDriveSiteUrl -LoginName $SiteCollAdmin -IsSiteCollectionAdmin $True
Write-Host "Site Collection Admin Added Successfully!"
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