Once an erudite has used convert spell to power, that spell now exists as a psionic power, even if only that erudite knows it. Which, RAW, means that it’s a valid choice for Expanded Knowledge—that feat says you can learn “any power.” The subsequent clause referencing other power lists is introduced by “including,” and so it represents a non-exhaustive example subset. So it provides no limitation on selecting powers not on other classes’ power lists. There aren’t even any rules requiring the character who takes the feat to demonstrate that knowledge of the power’s existence—though a DM could rule that a particular spell isn’t a valid choice because no erudite has ever converted it to a power. I’d certainly rule that way, because in my games erudites certainly do not have that ability in the first place.
I’d say a DM would have to be insane to allow that, but then we’re talking about a hypothetical DM that allowed convert spell to power in the first place, so maybe that’s precisely what they are. Nonetheless, there do not exist strong enough terms for me to stress how much I consider allowing spell to power to be a horrible mistake, and allowing access to the results via Expanded Knowledge is an even larger one.
Doesn’t help with learning the power, but just because it’s frequently forgotten, you can manifest other people’s powers, which would work for manifesting a converted spell if a friendly erudite was around to work with. Again, I strongly recommend that it be impossible to find such an erudite to make friends with, because I strongly recommend that no erudite have the ability to begin with.