Custom pages are out of scope for this (at least at this point), I COULD code it in but cost vs demand... There would have to be a way to add items into the map and since the current manner just auto-pulls data from the database there is no real "need" for the ability to add items (short of posting more content to your forums)
I do however have plans to write a separate sitemap utility for custom/standard pages; this script since i have to hop through hoops to cater to limitations (filesize and total links) it would be difficult to factor in custom pages, the next product will gear just towards your "standard site" items.
You could always try adding them to the sitemap manually (by hacking it in) so long as you're sure you won't bust over the 50k links/page limitation.
