I think it'd be better to bring them in earlier rather than later, for a couple of reasons.
One is that the only way to leave them out until a deal has been concluded among the existing partners is to make a statement of distrust, basically that we don't want China to break into our club at this stage. What will likely precipitate is an alternative trade agreement that China will need, which already is in the works—it's called the Regional Comprehensive Economic Partnership, RCEP. That creates the risk of another mega-regional agreement among only Asian countries that will potentially serve as a competitor, if you will, to the TPP and a fragmentation, I fear, of Asia-Pacific economic relations that will not be healthy for Canada or for the region as a whole.