You could left-click on the tab you want to merge, and then Ctrl-left-click all the tabs relevant in that workbook, then right-click and choose "copy" or "move", and then navigate via the drop-down box to the file you want them all to reside in. That can be one of your existing files, or you can choose "new" and move them all there. You would have to do this with each workbook that has tabs you need.

This can be a little complicated if the file types are different, i.e., .xls versus .xlsx, but is completely smooth if the types are the same.