I haven't stumbled upon any ideal way to do this. I have seen a few options:
Right-click each symbol and add to other watchlist from the menu
I think if you edit the watchlist and change its name it keeps the original and creates a new one with the new name. Then you can remove unwanted symbols. Check if it preserved the original list before removing symbols as I'm not sure about this. This obviously isn't a good solution if you're trying to merge more symbols into an already existing list.
Multi-select and copy/paste to Excel or other spreadsheet. Select the column of symbols and then in TOS, import from clipboard. You can choose to wipe the existing symbols or not when importing. A text editor can also be used instead of spreadsheet but then you have to edit to get rid of all the other fields leaving just the symbols. Unfortunately, copying from TOS and then trying to import from clipboard without some intermediate step doesn't work.