can i put a sim card into a card slot for internet?

Your best bet is to tether your iMac to a cell phone or a MiFi (pocket WiFi) device via WiFi. If your cellular service provider doesn't permit tethering, MiFi may be your only option short of a USB cell device.

Personally, I believe that WiFi is a much cleaner option than USB.

No.

You are welcome.

As stated you can't do that, however, most wireless providers

have solutions to getting wireless access via some sort of

adapter, usually USB based.. Check with your provider.