Recompile your kernel and add it:

CONFIG_NLS_ISO8859_1=y


What it's set to is defined with:

CONFIG_FAT_DEFAULT_CODEPAGE=850
CONFIG_FAT_DEFAULT_IOCHARSET="iso8859-1"

Make sure the default code page is there too:

CONFIG_FAT_DEFAULT_CODEPAGE=850
CONFIG_SMB_NLS_REMOTE="cp850"
CONFIG_NLS_CODEPAGE_850=m

I'm assuming you have already kernel auto module loading...