Generally speaking, is it not possible to indicate 'was' or 'were' on '是‘？
Think about it like this. Let's say we recap this conversation in English, but omit any tenses:


L: Yingzi, be not you working for a company?
Y: Yes, I be. 5 years ago, I be a sectary blah blah. Then I quit.


You would have no problem inferring the tense of these sentences, right?

Chinese language is just like that. It doesn't depend on using different forms of verbs to indicate tense. You just have to infer that information from the context. And put enough information in your sentences for others to grasp your idea. 

You may have seen something like 爱过, 爱了 in Chinese but they are not equivalents of "loved" in English. They are just 爱(love) with additional information 过 or 了 to indicate the action of love happened in the past or have been completed. There's no such a thing as past tense (or any tense) of a verb in Chinese.

This ambiguity makes Chinese language more versatile (and more difficult to learn).