I´m stuck here. I need to convert what I just drew with the grease pencil to curve/mesh (don´t know exactly the term) so I can connect it afterwards to the mesh above. I´ve been looking around and I found that maybe the convert button could help me doing what I´m trying to do, but as you can see it is disabled. How can I enable it? or if there´s a more effective way to do what I intend to, what is it? Thanks in advance for any help you can provide in this matter :)
You are in Edit mode of the mesh. If you want to convert Grease Pencil layer into curves / mesh you need to exit Edit mode of any other object. Or you can use Bsurface addon to convert GP drawing into the mesh right in Edit mode on the fly.
Simple Converting Grease Pencil drawing
Once something is drawn on the mesh, exit Edit mode of the mesh (the Edit mode of Grease Pencil may be left on) and choose Convert in the Toolshelf > Grease Pencil. You'll have options to choose that you want it to convert to - Path, Bezier Curve or Polygon Curve.
Convert Grease Pencil in Edit mode
Use Bsurfaces addon to use Grease Pencil as retopo tool and convert it into mesh surface while in Edit mode.
After enabling addon create some simple mesh, enter Edit mode and delete its vertices. After that, draw with Grease Pencil and once finished, in the Toolshelf > Tools > BSurfaces rollout press Add Surface to convert your strokes to surface.
The Bsurface needs either 2 Grease Pencil strokes (shown above) or one stroke and one selection of geometry (below). To make strokes drawn with Grease Pencil to connect to already existing surface draw a stroke, select the edge loop of that surface which is the nearest to the stroke and then proceed to Add Surface.
Watch the tutorial on more techniques about using this functionality.