As in more "experienced" customer service agent
The term 'senior agent' in the meaning of 'seasoned agent', which is what you seem to want, is not directly translatable into spanish.
Spain (this is old enough that it is also related to other spanish-speaking countries) has a learning system at work that makes people start as apprentices (Aprendiz) and then jump to fully featured workers as second-class handworkers (Oficial de segunda). These can, with experience, become first-class handworkers (Oficial de primera) with the same workload and kind of work. It just happens that they get more salary, and also that in the absence of a team leader they become the team's head.
Problem is that Oficial de primera is mainly used in handworking professions (builder, labourer, carpenter) but not in places where we would use 'agent' in English.
On the wild, I would use experimentado (seasoned) or superior.