Apparently, there no exact equivalent of “profesor agregado”.

In the academic rank structure in the Central University of Venezuela, the “profesor agregado” is a degree/rank between “profesor asistente” (assistant professor) and “profesor asociado” (associate professor).

Maybe “assistant professor, 4th degree” is an approximate equivalent.

Best wishes,
Manuel
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PROFESOR UNIVERSITARIO.-
Puede ser:

INSTRUCTOR POR CONCURSO.-
Es el profesional que participa en concurso y es declarado ganador por un jurado ad-hoc de la Facultad de Medicina. El Instructor no es considerado personal permanente o de escalafón dentro de la Universidad Central de Venezuela.

PROFESOR ASISTENTE.-
Es el profesional que asciende después de ejercer durante dos (2) años como Instructor, previa presentación de un trabajo de investigación que debe ser evaluado por un jurado nombrado por el Consejo de Facultad.

PROFESOR AGREGADO.-
Es el profesional que asciende después de haber ejercido durante cuatro (4) años como Profesor Asistente, previa presentación de un trabajo de investigación que debe ser evaluado por un jurado nombrado por el Consejo de Facultad.

PROFESOR ASOCIADO.-
Es el profesional que asciende después de haber ejercido durante cuatro (4) años como Profesor Agregado, previa presentación de un trabajo de investigación que debe ser evaluado por un jurado nombrado por el Consejo de Facultad, y debe ser Doctor en Ciencias Médicas.

PROFESOR TITULAR.-
Es el profesional que asciende después de haber ejercido durante cuatro (4) años como Profesor Asociado, previa presentación de un trabajo de ascenso en el que deberá reflejar toda la experiencia adquirida como profesor a lo largo de su carrera universitaria.

http://www.cmdmc.com.ve/leyes/documentos/pdf/baremoconcurso....

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4.2 Academic Ranks: Core Faculty

Tenure and tenure-eligible faculty members appointed to an academic department are the "core" faculty and hold one of the following ranks: Instructor, Assistant Professor, Associate Professor, or Professor. A description of each follows. (See policy 405.2 for a complete discussion of the criteria for appointment or promotion for these ranks.)

(1) Instructor.
Appointment as instructor requires, a master's degree or its equivalent and demonstrated ability related to the role assignment.

(2) Assistant professor.
Appointment as or advancement in rank to assistant professor requires a terminal degree or its equivalent; demonstrated ability in teaching, research, extension, or other qualifying work; evidence of scholastic promise; and evidence of progressive professional development.

(3) Associate professor.
Appointment as or advancement in rank to associate professor requires all the qualifications prescribed for an assistant professor; an established reputation based upon a balance of scholarship, teaching, and service; and/or broad recognition for professional success in the field of appointment.

(4) Professor.
Appointment as or advancement in rank to professor requires all the qualifications prescribed for an associate professor and an established outstanding reputation in the field of appointment.

Utah State University – Policy Manual – Faculty - http://www.usu.edu/hr/policies/401.htm
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PROMOTION FROM INSTRUCTOR TO ASSISTANT PROFESSOR - The candidate must have a terminal degree or its equivalent appropriate to the discipline, preferably the doctorate, as determined by the component, together with other professional qualifications and accomplishments, including teaching competence in the candidate's field of academic endeavor.

PROMOTION FROM ASSISTANT PROFESSOR TO ASSOCIATE PROFESSOR - Normally, the candidate shall possess the appropriate terminal degrees for the profession. In special cases, the appropriate terminal degree requirement will be waived for a candidate of sufficient stature in the field as attested to by regional and national recognition.

PROMOTION FROM ASSOCIATE PROFESSOR TO PROFESSOR - The candidate's professional accomplishments shall be of such high quality as to deserve the awarding of this highest rank.

Normally, the candidate will have served full-time at least four years at Texas Woman's University at the rank of Associate Professor and completed six years in the profession at the college or university level as Associate Professor.

Texas Woman's University – Faculty Handbook - http://www.twu.edu/facultyhandbook/b-promo.html