¿Is there any way to save all the post descriptions made on Atahualpa? And how to export to another theme o with a SEO plugin (like yoast)?

There is no built in way to do this.

If you have set the ATO->Configure SEO->Use Post/Page Options? to 'Yes', there will be a new custom field avaliable in the Editor for Post and Pages one which is 'bfa_ata_meta_description.

If you have filled this in, a new entry in the wp-postmeta table is created with the value 'bfa_ata_meta_description' in the column 'meta_key'. The column 'post_id' will be teh post it is associated with.

You could create an sql query to retreive them and possible use that output for some other SEO plugin but that is beyond the support of this moderator.

good luck and if you figure out how to do it, add teh solution so others can benefit from it.