I want to add more fields to be indexed by solr. How can I do that I am now using search_api and search_api_solr. I want more custom fields, to be specific, like the weight for a term referred by a node content type, to be indexed together the out-of-the-box filed(like content type, created, language, etc.). How can I achieve that? what is the hook or api to do that?
Go to admin/config/search/search_api and click "Edit" next to your Index and select "Fields".

Check the checkbox next to the fields you want indexed under the column "Indexed" and click "Save Changes" when you're done.