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you should be able to attach the route, let me know what you try and find

Seems to be working without issues. I imported the route tables and it came with inline routes. Then I created an individual route to replace one of the imported inline and it worked. Also I imported a single route , destroy it, recreate it and so far so good. So the conclusion is: I can create additional routes with the ec2.Route constructor and attach them to the imported route table without issues. If the route table has the same destination then it will overrride that route. If the route was imported into the route table individually then it will be updated as well.