One way to do this is to open both workbooks, select the sheet(s) you
need from the source workbook, right click on the selected tabs, click
move or copy, select the work book you want to copy/move the sheet(s)
to,  then click ok.  This will copy/move the sheet from one workbook to
another without deleting any information in the target book.  You might
want to try this with some dummy files first.

Hope it helps.