I have an iPad2 Wi-Fi without 3G. Is it possible to buy an adapter so that I can use a SIM card from my USB?
No. That's not possible to do. You can buy a personal WiFi hotspot with a data plan to use when you do not have access to WiFi, but you can't add a sim card with an adapter.
You can use an iPhone as a personal hotspot for the iPad or you can get mifi devices (which use sim cards) which act as mobile hotspots - do a search for mifi or mobile hotspot in your carrier's store.
Try tethering if your carrier supports it
The trouble is, even if you could somehow get your iPad to read a sim, the wifi only version lacks a cellular data radio.
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