Global Address List (GAL) and Offline Address Book in Hybrid Exchange with Cloud only accounts.

Create those 200 accounts with onpremise AD as users and enable remote mailbox (it actually set remote routing address  or target address on those accounts pointing to .onmicrosoft.com ID in cloud) and sync them to O365, this way they will get connected to respective O365 users and will be populated with onpremise exchange GAL as well.

Thank you for your response but in our case, we don't want to have their accounts on-prem, Why?

- because they don't have access to on-prem network and will not be able to change their password.
- Don't want to pay Server CAL licenses for users who are remote
- Don't want to purchase Azure AD Premium for password write back


Our goal is to have corporate users (who are in the network) to have their account On-Prem synced with AAD but have their mailboxes in the cloud (Hybrid). But frontline workers to have their account Cloud only.

Thank you

You will simply create those accounts in onpremise AD
It not means you have to pay CALS for them since you sync those accounts with password to cloud and exchange online use azure ad for authentication and not onpremise AD
If you concerned about password, you can set non expiring password
OR
There are bunch of free online password reset tools available for onpremise AD, you can use that

Else You can create contact objects in exchange onpremise and point them to Onmicrosoft.com target address to populate with GAL but they cannot be synced to O365 actual mailboxes and then its difficult to manage onpremise GAL as contact remains isolated

Thank you for response:
- do you have any Microsoft documentation referencing to what you said about not being required to purchase CALs for users who are just synced ?

- do you know solid tool that would allow users to reset their AD PASSOWRD.  main concern is securoty.

Thank you

Thank you Mahesh, very helpful!