I've been searching a kind of extension to upload customers in a bulk but I didn't find anything. Do you have any system to upload a lot of customers via csv, xls.. etc? Or there's an option that I didn't find?
Magento has a built in system for that.
System->Import/export->Import. For the field
Entity Type you have the option to import customers.
The only catch is that the file must be in a specific form.
To get the columns you need go to
System->Import/export->Export and export a sample file. Fill in that file and upload it.