It's $35 to add hopping to a ticket, regardles of whether it was originally a 1-day pass or a 10-day. So, to upgrade your 7-day pass to a hopper (even if you only want to hop one day), it will cost $35 + tax.
If you discover that you have days leftover on your pass and wish to purchase the no-expiry option, you will pay an upgrade fee based on the original number of days on the pass. So, even if there's only one day remaining on a 7-day pass, you will pay $55+tax to convert that pass to no-expiry. And you must do that within 14 days of the first day the pass was used. To me, it hardly seems worth spending the extra money to salvage a single day. Since a 7-day base ticket is only a few dollars more than what a 6-day ticket would have cost you, my advise would be to either use it for a quick visit to the parks on your first/last day or just allow that day to expire.