LM2575 - use fixed as variable
Yes, you could do this, but the results may not be all that satisfactory. Below is the block diagram of the IC: 



There are two resistors inside the chip for fixed versions. For the adjustable version, there are no resistors (R1  is open and R2 is short).

You might think you could simply insert a series resistor to increase the voltage (1.23V/K ohms), however the tolerance and temperature coefficient of R1 and R2 may be very poor (the ratio however will be very accurate). 

You could insert an LED or a diode or a zener diode in series. You could add a lower resistance network from pin 4 to the output/GND so that resistance of R1/R2 wouldn't matter so much, but that would waste power. 

If the increase you want is small (say increase 5V to 5.5V) it won't matter so much. If you don't care much about stability (say you're using a pot for a one time setup) it might be okay. But nobody is guaranteeing the accuracy or the stability of those resistors (only the ratio) so you can't count on much.