Hello,

It is not possible, if you set some user as a global admin, you give him an ability to access Site Manager, which is only one for all sites and it can't be separated for each site, whilst CMS Desk can be only for one site, so you can create a role, which will have full rights for CMS Desk and you can place your user into this role to make him pseudo-administrator for that site.

Best regards,
Jan Hermann