hi again,
I finally find how to do it and that's quite easy when you know how to do it. I post it here as it may help others having the same issue.

Retrieve the zip file you need from http://static.maploader.maps.svc.ovi..._file_here.zip

You can find the number of the required zip file(s) by editing the catalog-00.02.49.104.xml file in /home/user/MyDocs/cities/MapLoader/ directory.
For example, for whole Europe, there is a line in the xml file which is like <File Description ="Europe" Id="2" Zip="2.zip" Link="maploaderzip-00.02.49.104/2.zip" ZipFileSize="4226468773"> so the file I need for whole europe is at http://static.maploader.maps.svc.ovi...2.49.104/2.zip

Then unzip this file into /home/user/MyDocs/cities/diskcache/ overwritting existing files when needed.
Et voilà! Europe is available offline in Maps and Drive and I don't have any issue about missing roads anymore!
I guess you have to change the "00.02.49.104" numbers in the catalog file and the url link according to the maps version you have. Of course, this also works for individual country (example 21.zip for France)

You can also download the POI using the same URL but putting the name of the continent as zip file, for example http://static.maploader.maps.svc.ovi...104/Europe.zip for Europe and unzipping it into /home/user/MyDocs/cities/diskcache/ClientIndex/

The only issue that I have is that I don't get the list with downloaded maps with a little green tick on the side, but tht's not very important IMO. Also I d'ont know how it will work over next maps udpate!
Hope this help!
Krakinou