OK : here are all the steps I did :
1) Download miniCD
http://ftp.fi.muni.cz/pub/linux/opensus ... ot.ppc.iso
2) Boot it under OF :
 boot cd suseboot/inst32 video=radeonfb:1024x768@60 linemode=0
3) Make sure CD1 is in your Drive : "Back"
   Select language : "English"
   
4) Enable network driver :
   Kernel modules (Hw drivers) / Load PPC modules
    Select "via-rhine" with no extra parameter
5) Open network connexion :
    Start installation on system
    Start installation update
    Network
    FTP
    Automatic conf DHCP : Yes
    IP addess of the server : 147.251.48.205 (this is ftp.fi.muni.cz)
    User and password : no
    HTTP proxy : no
    Directory : /pub/linux/opensuse/distribution/SL-OSS-factory/inst-source/
6) YaST2 :
 language : english
 licence : ok
 install mode : update
 a few hw modules are detected (USB, ReiserFS, Ext3...)
 Then I can select the partition where Suse 10.0 is installed (/dev/hdb5).
 
And here is a problem I can not solve in the Installation Settings screen : the installation media does not seem to be recognized :
- System
 System : CHRP
 Processor : 7447/7457, altivec supported
 Main memory : 512MB
- Installation media : ??
- Update options : Cannot read installation packages from installation media. Media error ?
- Packages
  Packages to update : 0
  New packages to install : 0
  Packages to remove : 1180
  Total sizeof packages to update : 0
- ...
Any hints ?
I'm going to try with full install if there is no solution with update but I would like some advice before.