Can we export all our (existing) product keys, alter the data (add more columns of feature keys, fill in Excel etc. and create on LimeLM) and then import *overwriting* (or rather editing/amending) existing users?
No, you have to modify the keys in LimeLM either using the LimeLM UI or using the web API.
You could benefit from using the bulk editing in LimeLM. just select multiple keys and click "Edit".
Can't see how to set default - can it be set (on a per-feature key basis)?
It's coming.
Can we do that somehow? (maybe #1: i.e. export, alter [incl. delete/add feature keys with different data type] then (re) import?)
Add a new custom license field of the type you want, loop through all the keys and set that new custom license field to whatever you want, delete the old field, rename the new field.