It is possible to upload lists of companies into Bloomberg. You have to use the PRTU function (Create/Update Portfolios) on the terminal.
To create a new portfolio, click 1) Create in the red bar. Give your list a name and click the button 1) Create. A new screen appears, like this:
Then go to the excel file with the company codes. Select the column with the data and move the mouse over the top-right corner of the column: the pointer will become a cross with four arrows, like this . Then drag this into the PRTU function, to <Search>. The codes are put under Cash and the data under Descr, ISIN, SEDOL etc is populated. Check this data and when you agree with it, click 1) Save in the red bar.
You can use a portfolio for example if you are looking for Rating changes: type in RATC on the Bloomberg terminal and click 1) Searches in the red bar. Under Companies at the bottom of the screen you can mark Issuer List, select Portfolio and then pick your portfolio from the list. You can add other search criteria, like dates, agency or rating type. Click 1) Apply to run the search for rating changes for the issuers in your list.