It is possible to upload lists into Bloomberg. You have to use the PRTU function (creating & updating portfolios).
First create a new porfolio, give it a name. Save your portfolio. Next select your series in Excel by pointing the mouse over the top-right corner. The pointer becomes a cross with arrows which can be dragged into the PRTU function at row 6. Finish by saving your list. The size of a portfolio is limited to 2500 series.
Next use another function, e.g. rating changes RATC to use your portfolio. In RATC select other search criteria number 6 search by security list. At source, select option 6 porfolio and select your saved list. Finish with update.