If your report's recordsource is a saved query (that is, one that if you go into the queries list in the Access window and you can see it), then you would just select it and click Design and then, if the table is there that you can add it to the query, you can drag and drop it into place. If the table is not there, then you will have to click the Add Table button (see the + sign with the icon of a table) and then select the table you need. Then, you will have to link the tables, then add the item.
If your report's recordsource just starts off something like SELECT ... then you just click the elipsis next to the recordsource property and it will open the same QBE grid and then follow the rest of the above instructions. Then, when you close the query select YES to the question. Don't click the SAVE button until you've closed and said YES to the question of whether you want to change the SQL of the recordsource otherwise you will end up creating a saved query (which is fine if you want to, but you don't have to).