It seems like you ought to be able to from from the dialogue, but I haven't seen the option come up.
And if you can, how do you go about it?

Is this definite?

lalalala

[spoiler]
I kept getting the option for Esteban to join my expedition or at least ask him to, which I didn't because he had to die.
[/spoiler]

Yes it's definite.

And to add to the above spoiler somewhat (and yes GOG needs to add spoiler tags!) - Esteban will actually never, ever join you. Which is fine with me as I agree he has to die for what he did (or tried to do). >:)

Unless you're a kickstarter backer - then you can get him to join you. From what I've discovered he's just another soldier with standard soldier starting stats, although annoyingly he does have a cool/funny interaction available to him in Mexico.

An utterly defeated ex rebel leader joining *your* adventure expedition is a completely different scenario to *you* joining a subversive guerrilla group. It's definite.