The answer to almost all questions about unit testing is "decouple from dependencies".
Refactor your code to decouple the logic (which you want to test) from the interactions with with Excel.
Remember that unit tests should answer the question "does the code I am writing do what I intend it to do" and not "does my program work properly" - that's for ...
I don't know what "in an office setting" means here.
I use WinTask for lots of my testing.
You can find links to trials of it and other tools here. Perhaps one or more of these might help:
The perfect example of simple quality assurance tool that can be found in an office setting would be your computer's MS Office applications. It helps in ensuring quality of your paper works and help you finished faster. I found a thread as an example that can be very helpful. Here it is http://www.techyv.com/questions/quality-assurance-software-uses-msaccess
I haven't done this type of testing, what I would recommend is looking into how you can write office macros that utilise the functions of your add-in to test it's functionality. You should be able to add vba command buttons to a spreadsheet or document or the word toolbar that runs your tests.
I am assuming that as you are testing and adding in VSTO, you ...