You can either re-assign partition manually after import, or if this is a repeatable process it would be worth adding partition assignment rules.
If you go the assignment rule route, you'll want to have a field built (if there isn't one already) that indicates the partition. You'd include that value in the csv and then the MKTO assignment rules would fire on creation. You would also want to build a triggered campaign for changes in that field in case you needed to batch some leads at some point into partitions, or if the value gets set after creation from some other source.
Hope this helps!
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