Disney only allowed modifying Magic Your Way tickets, even non-expiring ones, within the first 14 days of first use of the ticket. So, if the ticket has previously been used more than 14 days ago, then no, days cannot be added.
If it hasn't been used, as Steve says, they don't offer the non-expiring tickets any longer. For a while you could still add days according to the last price schedule, but I don't believe that to be the case. My assumption from the question is that if it was unused and you wanted to add an 8th day, you'd be planning a singular 8 day trip. Using the No Expire ticket in a single trip is a very bad value...that ticket was worth about $585 at the last price schedule I had, where a brand new 8-day ticket is $420, not including tax (or options). A No Expire ticket is best used over multiple trips.
And if you wanted to just apply the value of the ticket to another ticket, you need to choose a ticket of equal or greater value like an Annual Pass. (Edit: At 2006 prices, the cash price was probably a lot less, so it could potentially be applied to a new MYW ticket...)