USB voltage amplifier
You can "amplify" a DC voltage, using for instance an op-amp. But that requires the op-amp to already have a higher supply voltage than the one you want to make.

Probably what you want is to convert 5V to 7V.

Converting DC to a higher DC voltage requires first turning the DC into AC, storing the energy in a capacitor or inductor, then rectifying and filtering ripple to end up with DC again. Keywords to search: smps, dc-dc converter, boost converter, buck-boost converter, or charge pump.

If conversion was 100% efficient (which it isn't) you'd still be limited to power in = power out, so the answer to "can I power more LEDs" is generally no. It might be more convenient to power some things (like LEDs) from certain voltages, but ultimately they require power.