Using USB to supplement solar panel
A diode, preferably one with a low forward voltage, probably followed by a large capacitor:







simulate this circuit &ndash; Schematic created using CircuitLab

Whichever power source is able to supply the higher voltage drives the motor of your pump. If the pump is off and the sun is shining, no current will flow, and C1 will be charged to your solar cell's full voltage of 7V.

However, as soon as you start drawing current, the voltage supplied by the solar cell will drop. For a long time, this will still mean the right side of D2 has a higher potential than the left side, so no current flows from the USB power bank.

However, as soon as you draw so much current that the solar cell's voltage sags below 5V, D2 starts to conduct and you start "helping" the solar cell from the USB supply.

This is not really a great arrangement: Much more elegantly, you'd build a solar charger for Lithium-Ion cells like in your USB power bank, and use the sun to charge that, and also supply to the motor. ICs to do that exist, it's just way more work.