There's a "replace scene" checkbox in the import panel. If it is checked, all contents currently in the scene will be removed.
Anyway, it's more likely you will want to import, polish up and pose characters a bit in separate .blend files first. Then when you have the characters the way you want them you can "append" the character into a third .blend file (from the file menu). When doing this, not that it is the "group" inside the blend file you append.