I don't know HOW you get into multiboot's terminal, but I know both version (bootmenu's and multiboot's) of BackupMenu have a terminal that you can get to.

They both ALSO allow you to resize your various partitions / make new ones, because the latest versions of BackupMenu include a bunch of system tools like that (as well as an ability to get in with SSH). At least, if I recall correctly.

As for how, I'm sure the multiboot threads should explain it.