Hi thiagonmat,
Thank you for your post and welcome to SEBLOD forum.
SEBLOD enable you to implement multi-category feature. However, it's not really multi-category. Why, because Joomla! does not allow do it. Each article is associated to only one category.
So, you can get around this problem with SEBLOD. Why ? You should for that associate all of your article to one category ok. You will find all of your articles in one category in the content manager.
But, with seblod, you will add an additional field called category that has another storage (for example: standard -> article -> category). In your form you can use checkbox or multi-select field in order to allow you to select several categories. Finally, you should define a search type in order to retrieve these article not based Joomla! Category but base your new category field.
Regards,
Mehdi.