What version of the JSS are you running? You mention site so I'm going to think version 9 (which I'm currently not using). So my knowledge is from version 8.
Are you able to do one of the following:
A) Perform an inventory search to get all of the machines you want to set to a specific location?
B) Already have your systems grouped within a Smart or Static computer group?
If you can do either, in version 8 you were able to display the list of results from the search or View Membership on a smart/static group and at the bottom you could "Take Action on Results" and then select Edit Building, Department, or Servers. This only allows you to set one of the values you stated, building. To my knowledge there isn't a method to modify the username (unless it's the management account). The site part I do not know, but according to 9.2's administrator guide on page 166 you can edit site in mass.
Beyond that, if you don't already group the systems you want to interact with somehow or can't group them in a search, then you're doing more of an interaction directly into the database based on your spreadsheet. It's never recommended to interact directly with the database unless absolutely necessary and I'm not an API guy at this point, but there might be something there more advanced folks could suggest.
