Can I run a liquid-pump motor below its specified operating voltage range?
N α K Eb/ɸ

Where 

N = Speed of Motor

Eb= Back Emf

ɸ = flux per pole

From the above relationships  the speed of DC Motor is directly proportional to Eb(Back emf) and inversely proportional to flux produced by field. (General DC Motor -Dc shunt)

So by reducing the Input voltage obviously( Back Emf) will reduce the speed of the motor. I am guessing there is no provision to control field.

Before running the motor in low voltage continuously for long time, check for the current taking by the motor. If it is more current than specified level then adjust the voltage such that it should not take more than specified level. 

More current cause to lead the winding temperature to predefined level so it will damage the motor