Are human Sith Inquisitors necessarily born as slaves, or is it possible that they were born as average Imperials and became slaves later on?

It is not specified in the story, and it's the same for all species of Inquisitor. (Caveat: Twi'leks get a few special lines in certain parts of the story.)

During an early conversation with Andronikos you can talk about your backstory, though the options available change i think depending on your earlier choices (I think in that first conversation with Kallig?). You can be a former slave, or a convicted criminal who got made a Sith instead of being executed.

So yes, it is possible to have a non-slave backstory.

During an early conversation with Andronikos you can talk about your backstory, though the options available change i think depending on your earlier choices (I think in that first conversation with Kallig?). You can be a former slave, or a convicted criminal who got made a Sith instead of being executed.

 

So yes, it is possible to have a non-slave backstory.

Is it? Doesn't the overseer on Korriban call you slave right from the start?

During an early conversation with Andronikos you can talk about your backstory, though the options available change i think depending on your earlier choices (I think in that first conversation with Kallig?). You can be a former slave, or a convicted criminal who got made a Sith instead of being executed.

 

So yes, it is possible to have a non-slave backstory.

To be honest, being a convicted criminal doesn't negate being a slave before you committed a crime.

The game makes it pretty clear over and over that you were a slave (it's stated in the opening scrawl, Harkun, Zyn, Ffon and Zash refer to you as a slave in the Academy, I think Skotia and Thanaton reference it later, the Inquisitor can express disgust for the concept of slavery or the word "slave" over and over, etc), but the details are left up in the air.

Whether you were from a family of slaves or made a slave when the Empire conquered your world, or whether your were passively discovered or discovered after you were about to be executed for committing a crime (in an Alien Sith Inquisitors' cases, "Some Imperials thought it would be fun to shoot my kind for sport. I shot back. They wanted to kill me, but they made me Sith instead) is up to the player's imagination.

You're also just telling Andronikos how you came to be Sith (i.e. when you were discovered to be Force-sensitive) and getting in trouble for breaking Imperial law sounds like a really easy thing for a slave to do.

So yes, it is possible to have a non-slave backstory.

No, it's not. That's literally what the Sith Inquisitor arc revolves around, namely that they're a slave and thus not worthy to become Sith (according to Sith traditionalists). You can choose how you came into slavery (from birth or later when convinced as a criminal), but no matter which you choose, the Sith Inquisitor will always be a slave in their background.

Man reading on the comments leads me to such a good nostalgia trip, its been a while since I started an Inquisitor Story... its so interesting and entertaining! (IMO)

God I wish SWTOR team release the game dialog script of the Inquisitor Class... I know it probably be a mess with so many options and variations of dialog depending on choices... but it be such a good way to kill time now that I'm working from home as I cant pause the game to relive the story in-game lol.