Yup!
Although it may not be the best idea unless you are somewhat familiar with it... I'll give you some details and you can see if you want to try, or go for something else...
Lithium Ion and Lithium Polymer are sensitive chemistries that don't like excess heat, and are very sensitive to over-charge and over-discharge... There are also well defined limits to the peak and constant draw of current from these batteries.
So, extra care MUST be taken to charge and use Lithium rechargeable batteries compared to NiCd, NiMh and Lead-Acid.
So, other than the Lithium part, lets talk about battery banks in general.
In parallel, their capacity is added together. (Both their current supply, and mAh rating...)
In parallel, you typically have a lower voltage, but a higher current. All of the cells in a parallel pack are self-balancing.
In series, their voltage is added together, but their current supply is not.
In series, you typically have a higher voltage, but a lower current. The cells do *not* self-balance, so there is a risk with under-charging or over-charging a single battery in a pack, even if the pack is not 'empty' or 'full' ... For a series type pack, it's a good idea to use more forgiving batteries, or you will find that the pack as a whole will degrade quickly and require that one or more cells need to be replaced. You can build a circuit to keep them balanced, but that's extra work
The more cells you have in series, the more likely this problem is to occur with repeated charge/discharge cycles.