I never used MySQL field so I don't know if it works. But you can't make CB directly use cb_peopletraderid to login. CB can only use username or email so you have to copy the value of cb_peopletraderid assigned to the user to his username. This can be done either after registration, after confirmation, after approval, etc depending on your membership model.
So IMO the process is:
1. User registers
2. User is assigned an ID
3. Set his username to this ID
4. You can tell him that he can login using his ID
You need to decide how you will generate this ID (incremental, random number, etc), when you want the ID generated (registration, confirmation, approval, etc) and where you want to store this value (jos_comprofiler, create a new table, etc).
Note that if you generate & assign the ID on CB Subs plan activation, a plan is active when payment of that plan is received (instant when using credit card/paypal or when admin mark basket as paid for offline payment) regardless if the member is enabled/approved if your membership needs admin approval.