No, it isn't.
But you are not going to particularly like the alternatives..
1. Air has no ethernet.. so buy the dongle.. usb to ethernet or better but uber expensive.. thunderbolt to ethernet.
Use the TC in second isolated network. Faster more reliable.. but in the end you could have done that with a USB drive direct.
2. Use a secondary wireless. Wireless has one big limitation.. it cannot be the client of two different WAP at the same time. (Ethernet btw can be clients to two different routers). But you can get minature wireless USB sticks.. they are now so compact it is not like some great dongle handing off your neat small laptop. Having a second wireless allows you to set up a secondary network. And directly link to the TC.
I have given the basics of the setup for either of these setups here.
https://discussions.apple.com/thread/4817218?tstart=30
3. Use a wireless bridge in front of the TC.
Here is the differentiation of products.. although neither a TC nor an Extreme can work in "join a wireless network" with ethernet active and express can. (You can of course choose any other wireless bridge device.. although you need to find ones that do universal wireless bridge because you will no access to the wifi or because the device is not built for bridging). You can then plug the Express in front of the TC.. you effectively capture the wireless, convert to ethernet.. so the TC can convert back to wireless.
4. If you own the myfi device.. throw it over the side, after you recover the sim, and buy a proper 3G or 4G wireless router with ethernet. This is IMHO the best option if it is available to you.
Edit..if the myfi device is your iphone you don't need to take the throwing over the side literally. Only if it is one of those cheap and nasty .. usually HuaWei supplied hotspot thingos.. Use a separate data account for 3G. If you are using wifi supplied by marina, then ask them what they would recommend as a matching device for ethernet connection.
5. You can also use a 3G modem dongle direct in the laptop. Some of the cheap ultrabooks are now being sold with 3G modem inside which is a great idea. Just slip in a SIM. Apple limit any kind of add on of course.. no hatches to make life easy.