If the lawyer has not been insured for that year, Marsh would advise the firm of the premium payable.
If the new lawyer was practising with another Firm prior to joining your Firm, there may be no premium payable since the lawyer is already insured for the policy period in question.
Whether premium is payable is dependent on when the lawyer left his previous firm. If the lawyer left before the inception of the policy period, his previous firm is entitled to a refund. As such, when joining your firm, there will be premium payable by your firm.
However, if the lawyer left his previous firm after the inception of the policy period, there will be no refund to the previous firm and his insurance will protect his previous firm for work the lawyer has done and will continue to protect the lawyer at your firm.