No, you cannot insert or update the data directly into the database table using stored procedure. The thing is, CRM will perform a bunch or related processes (e.g. created by, created on, update by, update on, triggers plugins and/or workflow, etc.). So, you should insert or update data through CRM SDK Web Service.
You can only QUERY against the database directly with FitleredViews (e.g. filteredaccount). Also, your stored procedure should not be placed inside the MSCRM database of you organization as this will populate the database and no gaurntees that your sp will be upgrade to the next version.