Bonjour Flavien,
You can simply define a grid with the proper size, and create your agents from your shapefile.
You can simply associate a grid cell with each of your agents (get the grid cell whiwh contains/covers the centre of your agents from shapefile) and assign the values from your shpefile agents to various attributes of the grid agent.
Note: you can also simply keep your shapefile agents (and do not use any grid ... )
Another way could be the save your grid in a multi-band format (such as tif files) and in your model create the grid from the tif file. I am not totally sure that saving a grid to a tif file will save all the bands , but it could be an improvement of GAMA to get that.
Cheers
Benoit