Publisher has to submit a formal request to the databases along with an application form. The database management would appoint a selection committee for evaluating submissions, if the basic requirements are fulfilled by the publications. This is the general process followed by the databases for journals. Hope more or less a similar process is followed for conference proceedings/book series as well. One can refer the following links to get an idea of the review process/selection criteria followed by Scopus and Web of Science.
Ashok Kumar Upadhyay This is a great question. It would help if you searched for journals that are already listed in the Web of Science. The Editors of some journals commit an issue or a Supplement to cover abstracts of conferences. Ask the Editor of such journals if they are willing to invest a Supplementary to your conference abstracts. In this regards, you need to search for journals that match with the theme/and topics covered in the conference. You may need to discuss with the Editor, the process of submission, and the costs that the conference has to pay to the journal. Plus regulations of the journal regarding this process.