You're really asking two seperate questions:
Is it possible to make an open season reservation using regular club points.
Open Season availability is basically the same as Club availability so your question could be phrased "can club reservations be made within 30 days of check-in?"--the answer is yes. The only difference would be that Open Season allows two night stays; in that case... no: rescued, current, or borrowed points may not be used for an Open Season two-night reservation.
I have a situation where I would like to extend our stay, but there is no availability now to extend the reservation. If at the < 30 day mark , there is an availability, can I modify my existing reservation to add an extra day?
Open Season reservations have a two day minimum; if you made a new Open Season reservation it would have to be for two more nights. You really just want to extend your existing reservation.
The first question is: is your existing reservation "changeable"? i.e. Did you make it online, or pay the extra $20 ($69 total) reservation fee to make it changeable? If so, then there is no problem (as a previous poster noted) you can just keep checking and if availability opens up you can simply add another day. The points would come from where they currently have priority in your account (I assume your rescued points).
One note: I would call in addition to checking online, especially if "Open Season" availability shows up and "regular" booking doesn't show it. The reason would be that club reservations require a three day minimum and Open Season only two--shorter than three nights would probably not show up in the availability chart for regular booking.
Also: if just one night opens up (which is exactly what you need) it may not even show up in Open Season. Just keep calling and you will probably get lucky.
Changeable reservations made online are supposed to only be changed online, but in certain cases where it is not possible (such as yours) they will be happy to do it over the phone.
If your original reservation is not changeable, then your only choice is to make a new two-day (Open Season, no rescued points allowed) or three-day (Club) reservation.... if it opens up.