Can a Site user enter orders in another site users account (impersonate)?

Yes, this is possible if the site user has been given permission. The access to another site user’s account can only be made from their (person impersonating) own storefront. They can impersonate if:

From the users storefront, they will go to: Settings > Users, and select the "impersonate" button below the site users name. When a user selects Impersonate from the list of users in their storefront, they will be logged out of their own account and they will be brought to the new user’s account without having to log in. Once logged into the other site users account, they will see all the same options that user would see themselves. If the person impersonating the user has different payment or shipping options available (as example) they should not be seeing those, they should only see what the impersonated user can see and order. If the person impersonating needs to impersonate other users accounts, they must return to their own storefront again and select the next user’s account from their list again.