It's definitely possible. He's not sure it would even be that hard if the power input were solved and if one didn't care too much about the exact final shape of the god at the end of it. Inconveniently, he does care very much about that.
Golarion solved it. Maybe not perfectly, Carissa knew rather little about the exact mechanism of the Starstone and Altarrin is incredibly unclear on what Irori did, but - well enough, it sounds like Aroden the immortal human and Aroden the god had recognizably similar goals.
(He has a lot of notes, from Arbas, on Carissa's rather confused early reasoning about truth-preserving minds - on her speculation that dath ilan teaches the sort of process that Irori followed in order to become a god. He's not sure how useful that direction is, yet.)
Fundamentally, the difficulty is that god-minds (especially Velgarth gods, swimming in Foresight with, he suspects, very limited sensory modalities to perceive reality on the level that mortals do) need to be very, very differently shaped from mortal minds. And the pieces of a mind are all interconnected, and most human minds don't have - stable invariant centers. By default, he suspects, if one part shifts then everything connected to it will shift along with it, not entirely predictably, and that includes the parts where values, goals, priorities are kept.
There's the thing he does, but it's hardly mathematically rigorous. It works, or seems to, but it's been tested only in a narrow range of use cases – he can transplant himself to a different human mind with different former innate traits, but he's never tried his method on a mind even as inhuman as say, a gryphon's. And it relies heavily on the fact that his implicit procedural memory and mental habits largely come with him, even if he retains only a few dozen explicit memories of events. The content of his core memories isn't nearly enough to specify him or his values, just - to tie together the other fragments, to reassemble them into the right shape around the core driving force of a promise he made to the world seven hundred years ago.
It does feel, subjectively, like that is a clearly definable core – and probably to Carissa as well, he just can't introspect on that one directly. The challenge is in actually defining it, in a way that isn't just robust to massive shifts around it, but is provably so. Because - once you start the process of turning a mind into a god, it's not easily reversible. If you get something subtly wrong, one, it might not be possible for a non-uplifted human mind to fully check it, since that require understanding something more intelligent than they are. And, two, even if you knew exactly what had gone wrong, you would now be in the position of trying to control an entity that was already much more powerful than you, and wouldn't necessarily want to be changed into something else...
Carissa thought that dath ilan taught reasoning methods that...would apply, at least to an individual trying to shape their own mind in ways such that the core of them could if necessary survive major transformations around it. It's - in a way a similar problem as how to hold the important parts of oneself constant through the smaller mental transformations of various cognition-enhancing spells. While, of course, not locking in the wrong aspects of one's thinking – it seems important that enhanced-Altarrin was able to recognize that the Empire had been a mistake, and that wasn't a change in his highest-level goals, only an update to his strategy. It...also seems important that Carissa, with her thoughts unobserved and an Owl's Wisdom, was able to recognize that Asmodeus was opposed to her true values. Who knows, maybe both of them are still wrong about things just as big as that, things that want a god they're planning to build or become to be able to notice...
It's dizzyingly hard to think about. Altarrin isn't sure he's ever faced an intellectual challenge that was so far outside his current skills. He very badly wants Carissa's +6 headband– actually, he wants that and Owl's Wisdom, so he can hold enough in his head at once to reason about this non-sloppily and catch himself when he's being sloppy anyway.
Well. Carissa said she needed a year or two to figure out how to make a headband with both. This is going to be a project of decades. (He suspects it would be centuries, if he were on his own.) He'll make what progress he can on at least sketching out what problems need to be solved, and wait.