Translation:She works as a secretary in a bank.
You wouldn't necessarily need it here as it's specified that "secretary" is a position within a bank, so it could be read as being a unique position and therefore drop the indefinite. I can't tell if the implications of this wording are reflected in the original Dutch or not, however.
The Dutch wording is also open for interpretation whether or not the position is unique or not. Aside from that, I would personally think secretary is generally not as unique as for example CEO, which can never be more than 1 person. All in all, I would say the "a" should be optional.
'Als' doesn't mean 'but' or 'and'. It basically serves as one of two purposes. One is as a conditional word like 'if' (e.g. "Als ik win, krijg ik een prijs" -> "If I win, I get a prize"). It is also sometimes used instead of 'wanneer' (e.g. "When I win, I get a prize"), although this wouldn't be an exact translation of the word 'als'. The other way 'als' is used is how it was used in this sentence, indicating a comparison or purpose (e.g. "Dat is hard als een steen" -> "That is hard as/like a rock").