Days cannot be added to the pass, but a prorated amount of the original purchase price can be applied to a new pass purchase.
This can only be done at the Guest Services windows at WDW. You only get credit for the unused park days--nothing for unused Plus options. So, if it was originally a 5-day PHP, 2/5 of the value would be applied toward a new ticket purchase.
If the pass was originally purchased from the
Disney Store, WDW or any other official Disney channel, you should get the full pro-rated amount as credit. If it was purchased through a ticket broker like Ticketmania or Florida Orlando Tickets, you will only get a prorated portion of the wholesale value of the ticket applied. Those vendors make bulk ticket purchases at reduced rates and apply their own markup to turn a profit. You don't get credit for their markup.
Finally, since you can only upgrade at WDW itself, your new pass will be charged at the higher "gate" prices--not the advance purchase prices.
As you might imagine, it's usually better to just keep your existing passes and purchase new ones for the additional days you need. If your inlaws are going for 4 or 5 days, let one of them use both passes and advance purchase a single new PHP for the other. If the trip is any longer duration, then let them each use-up your passes and advance purchase 4 or 5 day PHs for the latter days of the trip. Upgrading will likely cost you more.