You can put styrofoam sheets in the microwave

Can you use a “kosher” microwave at work?

This is a pretty broad question, and it seems like you would seek advice rather than a strictly halachic perspective.

In general, there shouldn't be a problem if the microwave is definitely only used by Jews and only kosher food is placed in it and it is only labeled as dairy or just meat. The main reason I say "Jews only" is because there might be a problem from "Bishul Akum" even if a non-Jew puts kosher food or even a potato in it.

In practice, however, I would not recommend doing this in a mixed environment. I find most workers are both careless and "carefree". Non-kosher foods are likely to end up in the microwave by mistake or even on purpose. There are many Gentile and Jewish anti-Semites out there wanting to cause a problem and this is an easy way to demonstrate it and no one will tell the difference. When you're not there to watch, you've cooked your food in your oven and you've already eaten it. Basically, it is highly unlikely that anyone there will guarantee that the microwave will remain kosher unless a reliable Jew watches over everything that enters it.

Case in point: someone I know works at Price Waterhouse. He says that about 80% of the people on his floor are Yarmulke / Hut and / or "Zitzit-swinging" Jews. Despite this overwhelming majority, they will not put a kosher microwave in their pantry, largely for the reasons I have mentioned, and even among religious Jews, no two people have the same level of kashrut.