How do I add another Apple ID on my iPad?

You won't be able to add another Apple ID to your iPad, but you can create a new one you can use from your iPad.

See How to create a new Apple ID https://support.apple.com/en-us/HT204316  and

See the data implications below.

Some of these things may apply even though not changing an Apple ID: What to do after you change your Apple ID or password https://support.apple.com/en-us/HT204071

 Keep in mind that some things available under a first Apple ID will not be available under a new Apple ID. For example, you won’t be able to transfer anything purchased from the App and iTunes Stores to the new ID. And, there are other issues to be addressed.

And see what Eric Root says on this matter here  https://discussions.apple.com/thread/251483565 and

what Winston Churchill says about it here: https://discussions.apple.com/thread/7452747

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But, despite any other data loss you can set up family sharing between the IDs to share purchases made with them. See:

Family Sharing  https://support.apple.com/en-us/HT201060

Set up Family Sharing https://support.apple.com/en-us/HT201088

You can use another ID in different places than the current ID, but you can't have two iDs signed into everywhere. For example, you can have one ID signed into iTunes and a different ID signed into iCloud, but you can't have 2 different IDs signed into iCloud at the same time.