For adding onto existing claims, it's pretty clunky, you don't have to start over, you can do /land claim load (id), /land claim start then /land claim end followed up by /land claim grow (direction) (number of blocks) then end it off with /land claim save (id). It'll look weird because there'll be multiple squares of claimed land, but it all falls under one claim id, and for claim granting, you need to run /land claim load (id) then /land claim grant (player) (permissions), for permissions you'd typically do "all" unless it's a public store or something like that, you can do "all" for the player section of the command I believe, which would tie into making a store or something like that, but once you've done all that, just do /land claim save (id). In my opinion the land claim commands are pretty bloated and a bit annoying to work with.