That is a little bit tricky to do in Bob as the default Employment table will only allow you one "official" contract.
This will always have to be done manually, because the system doesn't allow you to create fields that behave like the default "Effective date" where it will add and "End effective date" when you add a new row. But as a workaround, we can create a new table within the Employment category, where you can add the relevant fields, similar to the default Employment.