Lucy,

After reading thru these Wikipedia articles you will be able to make a decision as to what to use there in your translation.

In both the U.S. and U.K. , VISITING PROFESSOR is what sounds to me to be the direct equivalent for PORTUGAL's "assistente convidado".

http://en.wikipedia.org/wiki/List_of_academic_ranks

Portugal
[edit] Academic ranks
Professor Catedrático, equivalent to Full Professor
Professor Associado, equivalent to Associate Professor
Professor Auxiliar, equivalent to Assistant Professor
Both Professor Associado and Professor Auxiliar may have Habilitation, while that extra degree is mandatory to all applicants to the rank of Professor Catedrático. This is an additional degree to which one can apply to usually years after the PhD (similar to the French Agrégation). Agregação is a two days exam based on curriculum vitae evaluation and a public lecture, where the candidate is evaluated by secret vote by a large number of other full professors where the Professors insert a white or a black sphere into a bag according to his or her decision.

The 3 ranks of professors may also be held by invited professors, according to the rank, coming from out of the university.

Portuguese universities still have some few other teaching staff in two ranks not holding a PhD. These two ranks have been extinguished in 2009 and will vanish after all the ongoing contracts terminate in the next few years.

Assistente, equivalent to Assistant Lecturer (without a PhD), normally someone who is simultaneously doing a PhD and holding a master's degree. Only teaches practical (or lab) classes.
Assistante Estagiário, equivalent to a junior teaching assistant. It used to be the start of the teaching career until 2009 and already extremely rare to find since the Bologna agreement. They are typically graduate studentss and only teach practical (lab) classes.
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United Kingdom
[edit] Academic ranks
[edit] Teaching, research, and scholarship career pathway
Professor
Reader
Senior Lecturer or Senior Teaching Fellow (or Principal Lecturer in post-1992 institutions)
Lecturer, or Teaching Fellow (or Senior Lecturer in post-1992 institutions)
[edit] Pre-/non-career grade
Assistant Lecturer, Demonstrator, Seminar Leader, Associate Lecturer, Graduate Teaching Assistant,
Visiting Professor Usually unpaid; distinguished individual with formal input into education and/or research, allowed to use the title 'Visiting Professor' (but not 'Professor) for the duration of formal arrangement.