Hi Ambi
I can't speak for Paraguay, but in the UK and Ireland becomming an AFISO is works along the same lines as becomming a controller - you don't go and get a licence off your own back and then job hunt with it; it's always done through the airport that needs the AFISO.
In other words, you first need to find an aerodrome/airport that needs an AFISO, and then go and apply for the job. In many smaller airports the AFISO role may also be combined with an ops officer type role, ie. when your not sat in the tower, then you're helping out with other areas in the airport, loading, fire and rescue etc.
Nice work if you can get it, I would think with your background in flying you have the basics covered many times over but would just need some brain-training in how to think like a controller / AFISO. Not that AFISO's 'control' as such but there are times when you will have to nudge the arrival flow in the right direction.
Good starting point would be a phone call to the relevant Airport's Ops Manager, mention your flying background and I would think you would have their attention straight away. Good Luck!