If I'm using an offshore broker that doesn't provide a 1099-B, I can supply some of the basic information required but what do I do when it tells me to upload my 1099-B?

If you don't have a 1099-B to upload then you'll have to provide the information on each individual transactions yourself. When you begin the Investment Income section of the return you'll be asked if you received a 1099-B, and if not you'll then be asked how you want to enter the transaction data.