only option is the dual batt system but as far as i know it will only work with oem bikes and motors that support that option.
you can buy the cables ect if you want to try it but you will still have the problem of mounting the 2nd batt on the bike.
tho i have seen you can add more cells to a oem batt and cut a hole out of the top to add more cells then add a custom cover on top, jimmy at insat might be able to help out there but wont be cheap and void any warranty.
other option is a back pack.
https://www.e-bikeshop.co.uk/XLC-Ebike-Back-Pack-Bag