Their website says:
Season ticket holders, Valley Gold members and existing Red Card holders are eligible to purcahse (sic) tickets for the game and seats can be bought by visiting the Valley’s ticket office in person or by calling 03330 14 44 44.
So it'll take a change in policy to get in.
At risk of repeating myself, unless Charlton change their position, you won't be able to simply turn up and pay to get in the home sections. You won't even be able to buy a ticket for the home sections beforehand unless you're a member there - Red Cards, I think they call their cards. They're...