Can we engrave iPad after bought form storefront?

No. You would have to order it with engraving at the time of purchase.

Custom engraving is offered for purchases made from Apple's online store at time of order. Devices are engraved during manufacture - and are shipped directly to the customer.

Apple Stores do not offer post-manufacture engraving services.

Be aware that if you have your iPad engraved by a third-party, you will very likely void your device warranty.

You need to ask Apple and they will not see your question in this user-to-user forum. You can ask at the store or:

Contact Support

there are stores that offer engraving of all kind of stuff you bring it and pay them to engrave

apple products do not require some special engraving technique