Hello Edward,
No, these chargers can not be stacked. They are inherently buck regulators and will need a consistent ground between the input source the battery cathode (negative terminal).
Most of TI's chargers are configured to step the input voltage down to the battery voltage.
With a 5V input you would first need a boosting stage to create an intermediate rail at a voltage above the stacked regulation voltage of your pack. The next stage should be one of the many multi-cell chargers that TI has available.
I would recommend looking into the WEBENCH tool to design the boosting stage:
http://www.ti.com/lsds/ti/analog/webench/power.page
The using the parametric search select the right multi-cell charger:
