I already have Office 365 in my Microsoft Devices, can I add Office 365 it to my IPad Air 5th generation

Your existing Microsoft Office 365 subscription can be used with your iPad. There is now an Office365 App in the Apple App Store both iPad and iPhone.

• Microsoft Office

https://apps.apple.com/gb/app/microsoft-office/id541164041

The Office 365 App for iPad is a “cut-down” single install App; this App does not replicate full MS Office functionality. But this is not the end of the story. 

There are individual Microsoft Apps for Word, Excel, PowerPoint, Outlook etc. intended for iPad. iPad models with a screen size of less than 10.1” can access a restricted set of App features, including creation and editing of documents, for free. 

For iPad devices with a screen larger than 10.1”, the MS App’s require an active Microsoft 365 Subscription - which you are already likely to have. If you don’t have an active O365 subscription, functionality is limited to opening and reviewing existing documents.

All the Microsoft Apps for iPad can be found in the Apple App Store.

Microsoft Word, Excel, PowerPoint, Outlook and others are all available from the Apple App Store. All require iOS/iPadOS13 or later.

• Word

https://apps.apple.com/gb/app/microsoft-word/id586447913

• Excel

https://apps.apple.com/gb/app/microsoft-excel/id586683407

• PowerPoint

https://apps.apple.com/gb/app/microsoft-powerpoint/id586449534

• Outlook

https://apps.apple.com/gb/app/microsoft-outlook/id951937596

• Teams

https://apps.apple.com/gb/app/microsoft-teams/id1113153706

• OneDrive

https://apps.apple.com/gb/app/microsoft-onedrive/id477537958

• Authenticator

https://apps.apple.com/gb/app/microsoft-authenticator/id983156458

• OneNote

https://apps.apple.com/gb/app/microsoft-onenote/id410395246

In summary, MS Productivity Apps for iPad (such as Word, Excel, PowerPoint) offer limited functionality without an Office 365 Subscription - but only for devices with a screen size of 10.1” or less. Devices with larger screens only permit you to view (but not create or edit) an existing document. Full App functionality requires an active Office 365 Subscription.

I don't know all of the Microsoft 365 plans but Microsoft 365 Personal allows 5 devices for one person. See: https://www.microsoft.com/en-us/microsoft-365/buy/compare-all-microsoft-365-products-b

You should be able to download the apps (Word, Excel, etc) from the App Store on your iPad and then sign in with your existing subscription if you aren't exceeding the allowed number of devices.