Make a 9v battery power USB powered light?
To power a 5 V device (LED lamp) with a 9 V battery, you need to look at the following:


The LED's current consumption at 5 V.
How long do you want to run the lamp ? This will determine the current rating of the battery you would need. 


eg: If the consumption is 200 mA at 5 V and you want to run this for 5 hours the total charge would be 1000 mAH (milliampere hour). You would then need a battery which is 5 V, 1000 mAH. The total supplied power is 5 Watts.

Now coming to your requirement of 9 V battery. 
You would need a voltage converter (ideally a Buck regulator) which can convert 9 V to 5 V at 200 mA. Choose a converter for about 500 mA for getting the max efficiency. Take 90 % efficiency for this conversion.

So, about 0.6 ~ 0.7 (750 mAh) battery would do. Try putting AAA cells. preferably. NiMH (if you want to charge them again). Please see that NiMH cells are 1.2V instead of 1.5V for regular cells.

If you are still intending to use 9 V (6F22) batteries, you might want to put more than one battery in parallel. 

Also watch out for the economics as the 9V batteries aren't that cheap !