You can find a lot of information searching for "build your own power bank" on the internet.
The gist of it is that your power bank needs to have a voltage regulator to create a 5V output on a USB port. You can then use the standard USB charge cable to charge your smartphone.
You will also need a charge controller to be able to charge the batttery pack within your power bank from e.g. a mains power source.
Possible? Yes, see above.
Effective? Yes, if you use a switch mode voltage regulator. A linear regulator could be used here, too, but with less efficiency.