Are "fixed voltage" LDOs adjustable too?
You can adjust a fixed regulator, but only higher than it's nominal voltage. For example if you have a 5V regulator, you can only adjust it to > 5V (assuming no negative voltage used)

A fixed regulator is just like an adjustable one but with the divider resistors internal to set the voltage.

The LM7805 datasheet has an example of raising the output voltage:

  

Any standard linear regulator (by this I mean those that use the most common 78xx type topology - there are others that don't, and have e.g. voltage set using only 1 resistor) is adjustable by altering the reference pin voltage level in some way (yours included - it's just a "copy" of the LM7805, so you can use these kind of circuits - see figure 12 in the MC7805 datasheet). For example, you can also drive it directly with an opamp: