ANSWER: Yes, it is possible for a client to sue an employed engineer and not necessarily their employer. For instance, if an engineer allegedly performs a service negligently, the person claiming to have suffered harm as a result of the negligent action has the option of suing either the individual engineer or the employer/firm.
Whichever the case, the question of who signed or sealed the drawings, plans or specifications pertinent to the project where the negligent action happened is not considered as relevant to the issue of who is responsible for the negligence.
The courts are more concerned about establishing if the person (engineer or firm) owed a duty to the individual/individuals who suffered damages and whether the engineer/engineers breached that duty and therefore caused the damages in whole or in part.