Open the first list in datasheet view, or export to spreadsheet. Copy all of the records.
Open the second list in datasheet view or export to spreadsheet, and paste the records in.
If the lists do not match:
Copy list A into Excel from datasheet view. Open a new Workbook in Excel, then paste the data in but when you paste it, select EDIT then PASTE SPECIAL and paste VALUES only.
Move the columns around until the list columns match the order of the List B columns (leave blank columns to account for any LIST B columns that are not in LIST A - and delete LIST A columns where LIST A column has no matching LIST B column). Export List B into Excel, into a new workbook. Go to the last row in List B. Open List A's value pasted workbook, copy all of the columns, go to List B workbook, paste it in. Close Excel.
Now you have all of LIST A items added to LIST B.