This is possible in the bookings area of the booking.com extranet.
The booking.com CSV file does not specify the booked room or how many rooms are booked for each booking, you must add this information to the CSV file before importing it.
Open the file you downloaded from booking.com in a spreadsheet program.
Go to the first empty column, usually column J and the one after "Commission"
On the top row add the word "Roomid" as the column header. The Column headers (row 1) should now be: Book_nr, Book_date, Booker_name, Guest_names, Arrival, Departure, Status, Total, Commission, Roomid, Email (optional)
For each booking, add the room id number to the new column called Roomid.
If it is a one room booking, this will be a single number of one of your room id's.
If this is a multiple room booking enter the room id of each room this booking should be imported into, separate the room id's with a space. If two of the same room type are booked enter the same room id twice.
Save or export the CSV file using the instructions for your spreadsheet program. The file must be saved in CSV format with comma separaters in the same format (plus added room id) it was downloaded from booking.com.
Make sure the date format is not changed, it must be in YYYY-MM-DD format.
Upload the file WITHOUT ticking the "Save Changes" checkbox. A report will be shown of the information read from the file. If the preview shows errors fix them in your CSV file and upload again.
Bookings with the Action "Add New" will be imported, otherwise the Action value will report the reason the booking can not import.
You should only use this function if you are comfortable working with CSV files.
It is a good idea to test with just a few bookings and upload a maximum of 50 at a time. It can take up to a minute to process a file, do not re-click submit while the file is processing or you may end up with duplicate imports.
Once bookings are uploaded they can only be deleted manually so make sure they are correct before making a bulk upload. If you upload the file twice you will create duplicate bookings.