The table says “+1 level of existing meldshaping class.”
But wait! you say. The text trumps the table!
Yes, that is true. But only if the text and the table contradict one another. Do they?
The text says “a meldshaping class to which you belonged prior to gaining the soul manifester level,” which is not nearly as explicit or unambiguous as one might like. Almost none of the words in that phrase have explicit game-rule definitions, so we have to use their general English-language meaning. In particular, the word prior can and is used to mean either “at any point prior” or “immediately prior.”
One of these usages of prior, namely “immediately prior,” is compatible with the table, and is in context the far more intuitive, expected understanding. It matches both the table and other classes that do similar things.
While rules-as-written as an interpretive style is happy to accept some very counter-intuitive, tortured readings of the rules, it does require that you consider all the context that is valid under the rules. In this case, for example, the table: though tables are always secondary to text, they still are rules sources and cannot just be ignored because you like to. Text only trumps the table when it explicitly contradicts the table (cf. rainbow servant, which explicitly says “each level” progresses a class, in contradiction to the table which says only six of the ten levels do).
Finally, stepping away from rules-as-written for the moment, the answer is No because to do so would be highly overpowered. While this system is far from any perfect paragon of balance, it does try at least somewhat to maintain some balance. Some prestige classes do give “something for nothing,” but none1 are so far gone as to give you half of an entire class for nothing. The soul manifester class requires levels in incarnate as a cost to entry: a psion cannot get 10 levels’ worth of incarnate meldshaping without giving up at least one level worth of psion manifesting. If the psion is unwilling to give up his manifesting, then that is what the Shape Soulmeld and Open Chakra feats are for.
If you are dead-set on having your cake and eating it too, then we are not the people you should be talking to. No one here can say anything that will make this a good idea for a DM to allow. So don’t ask us; ask your DM. Instead of trying to come up with evidence to demonstrate that this is either balanced or sanctioned by the rules (neither of which is true), you need to be honest and up-front with the DM that you want to do something overpowered. Some DMs run intentionally-overpowered games where this may be fine. You can look for such a game if you wish. But if you convince a DM to allow it and haven’t honest and up-front about it being overpowered, then what you have done is extremely rude. If it were me, and I found out you had done this even after being told that this would be overpowered and you kept that from me, that would be the end of any gaming between us.
- Technically, the anima mage does give you a quarter of a class for nothing, RAW, but that shouldn’t be allowed either for the same reasons.