You can create all the shares with our API that would save you time but not sure about adding each user to a specific share - list of all our API commands are below that you can use.

nas_share_access_afp - Modifies AFP share access.
nas_share_access_ftp - Enables and disables access to shares via FTP
nas_share_access_http - Enables and disables access to shares via HTTP.
nas_share_access_nfs - Enables and disables access to the given share via NFS.
nas_share_access_smb - Modifies SMB/AFP share access.
nas_share_create - Create share on specified volume.
nas_share_details - Display detailed configuration of share
nas_share_edit - Changes share location or comment.
nas_share_groups - Groups manipulation functions.
nas_share_list - Lists shares
nas_share_remove - Removes the given share.
nas_share_users - Users manipulation functions.
nas_user_add - Create user in the system.
nas_user_groups - Adding and removing users to groups.
nas_user_remove - Removes the given user from the system.
nas_user_rename - Rename NAS user.