Many thanks for the info Whistler, now I finally succeeded in porting Blue Shift to Steam.
For those who are interested you need to use Half-Life's game DLL and client DLL, replace monster_rosenberg with monster_scientist (all that the Dr Rosenberg does is scripted, and the skin of the guy is in the scientists model, so I wonder why the heck they needed a monster_rosenberg ?), and in order to get the armor and helmet replace all the item_armorhelmets with item_batteries - all you need then is to change the w_battery model with the kevlar+helmet one
You can then change the value for the batteries and the color of the HUD to blue and voilą, Blue Shift runs as a HL mod with the Steam engine !