When you are importing via code (which is practical necessity for large imports) you can supply import_id argument to wp_insert_post() to be used.
There is however no clean/safe way (that I can think of) to specify or modify post IDs manually.
In your case (if you are really stuck with manually created posts) I would add IDs as post meta through editor and when site is hit via old format link extract the ID and look up via meta query if there is matching post to redirect to.