Open the file, R-click on the route name in the route tab on the left side, then Properties, Via points. You'll see a list of waypoints or via points in the route. L-click top of list, hold shift then L-click the last one (highlights all). Then use the copy icon in the tab to copy to the clipboard. Open file (or click the route) that you want to paste to, Properties, Via points, then paste the list into the 2nd route. You can paste it at any point by highlighting the via point where you want it to be inserted.
Be careful about making one huge multi-day route though - you're better off having multiple short routes. They can all be in one file, so you can load them in one hit.