Not a BigCommerce user myself but just try to give you my though. Hope it will give you some directions.
There are two options you may try. ( Both require a bit of programming I guess)
Option 1: Leverage tableau web connector
It looks like a sleek option.
Option 2: Build a mini database in-house to hold the extract and merging data from BigCommerce.
The process would be:
1. Download data from BigCommerce,
2. Automate the merging process
3. Import that into the reporting database.
Then, use Tableau with the reporting database.
This is the concept of data warehouse that I use at my work. (I use Microsoft MSSQL database. But if you want open source check out Talend with Mysql for example).
The drawback of this option is that your reporting database is not live. Depend on your scheduling, it could be 1 day or few hours behind.