May I know if there are ways and also possible to use workflow or SPServices to temporary give a particular user (who does not have any access to the site) permission to read / write a particular items in a SharePoint 2007 List?
Assuming the "current user" is the one who has limited access...
Using SPServices.. no because you can't do any sort of impersonation (unless Marc decides to add some complementary back end code as well).
Using a workflow, yes but you would need to write a custom workflow activity that can manage the permissions.
You can with Nintex Workflow by using their call web service action and supplying a username and password.
You can't add users to groups natively in SPD workflows. As Kit mentioned, SPServices runs under the context of the current user so they won't have the ability.