Indico 2.7 does not exist, i guess you mean Indico 2.3?
Anyway, the only way to import data into an existing instance is to get the old data into a new 3.2 instance, and then using the indico event export tool to export it, and indico event import to import it into your existing instance.
Import event from Indico 2.3 in 3.2 is about the exact same thing; you may find some more details on how to best do it in there…