Could you clarify: do you want to read/write NTFS etc. volumes through the file manager or do you want to add other operating systems to the GRUB bootloader?
You should tune your /etc/fstab file which is the one that automount any volume/partition at system boot. There are options to specify read-only/read+write permissions (usually: -o rw for read+write, -o ro for read-only).
Some valuable help can be found in manpage of "mount" command and "fstab".
As SliTaz does not have the man command, you should get an online version of the manpage (google it, it is very easy to find).
First step is definitively peaking a look at
1 ) your fstab file: cat /etc/fstab
2 ) your partition map: su fdisk -l parted -l exit (and, of course, identify each partition as what it is use for: root of OS, home partition, simple data partition, /user partition, etc... it depends of the way you have configured SliTaz installation on your harddrive(s))