Limitations on sharing internet from public wifi

I don't have one near me now to test, but I seem to recall that if I connect via my MBP to a public wifi (coffee shop, airport, the usual) that I cannot share that internet connection via my MBP to other computers or a phone. However, if I connected over ethernet I could do so.

My question is if I connect using the MBP and a wifi USB dongle (like the BearExtender), can I then share that connection via the MBP's airport card?

Also, is there a way to use the Airport Express to share the connection (note I said share; I wouldn't be using it to receive the public wifi, I'd be using the USB dongle for that because it has a much better antenna). For example, can you connect an Airport Express via ethernet to a MBP and share the internet connection?

Or make a LAN at least?

Or would I have to 1. receive the internet via the USB dongle, 2. share that connection via the MBP, 3. have the AE receive the wifi signal from the MBP and extend it (act like an access point)?

If it helps, the situation is that with a USB dongle I can receive a rather distant wifi signal at campgrounds, but our phones can't get it. I want to use the MBP as a sort of router to accomplish that. So signal goes from distant public point to USB dongle to MBP to either iphones, or to AE and then to iphones.

Thanks,
Rob