4s2p lithium ion battery charging from USB
You can certainly charge a battery like this from a USB port -- all you need to do is boost the voltage to the appropriate charging voltage.

With a 4s pack, the trickle charge voltage should be approximately 4.2V * 4 or 16.8V. So, you'd have to boost your 5V supply up to 16.8V, which isn't an extraordinary demand -- a boost controller IC or module can do this for you.

Here is where I think you may need to consider practicality over feasibility (e.g. yes it is technically possible, the best kind of possible :), but is it practical?). If you want to charge from a 'regular' USB port, they are specified for 5V @ 0.5A, or 2.5W. When applying 16.8V, 2.5W means you deliver a current of approximately 148mA. It will take quite some time to fully charge 8 cells at 3000mAh each, considering you can deliver at best (neglecting conversion losses) 2.5W.

So yes, it's possible to do what you ask, but I don't know if it's the most practical or not.