Here is how I would pull that off:
First, make sure on import that your node/content ID's start at a number HIGHER THAN your old ID's. You should make 2 new columns in the DB of your new platform. Call the columns something like "old_node_id" and "old_seo_url". Prepare an import using the schema/mapping of your new platform, with the addition of those 2 extra "old columns" from the old content. Make sure only the old content has data in those cols. You can use "0" for the old_node_id if it doesn't and you can just leave the old_seo_url col blank if none set. Now you will have a functional listing of content with the addition of 2 trigger columns that will be used to deduce whether an A) node ID and/or B) seo url rewrite is needed.
Next, depending on what platform/language/structure you are using, you need to deduce where to drop a router script. Sometimes this is in a header controller. Sometimes this is in a seo/url/request/response class. Sometimes this is simply in the index file. Or sometimes its nowhere useful and 404 is the best.
Unfortunately, it seems that your old platform (WP?) doesnt use querystring to identify ID's, but that's ok. What the script needs to do is first look for the combination of existence of old_node_id and part of the seo url in the new URi. You can use $_SERVER constants (or whatever lng/codebase needs) to do this lookup within the URi string. If it matches anything in those 2 new columns you added, forward the request there.
BTW: If you get mega traffic, you should def autocache these maps as they are found, use a huge TTL so they dont need to regen.