How to convert year to date with Migrate
Just found out that to_format value is the problem. Setting it to 'Y-m-d' works fine and imports the year value as date putting month and day to values of today. So actually to get specific day and month added to year value, this code works fine:

process:
    field_fecha:
      plugin: format_date
      from_format: 'Y'
      to_format: 'Y-01-01'
      source: year


Probably that's because I defined the date field as "date only". For "date and time" fields 'Y-m-d\TH:i:s' should work fine.