Can I import just part of my data?

Yes, but there is a downside to be aware of.

Importing a sub-set of data, as opposed to the entire set, means analytic tools like graphs and stats become "less true" in your overall account. 

For example, say you import data related to a single payee and exclude data having to do with others, because you already paid the others out for it. From an analytics perspective, there is data "missing" from your overall catalog income total - the reality is there was more income, it just wasn't added to IC. Nothing will be *wrong* from a payee balances perspective, it's just that there is some more "noise" in the data. 

Instead, you could import the full set, along with transfers to account for the payee balances you already paid out. That way, you'd have the full data set from an analytics perspective, and the balances would still be correct - you'd also have a record of those payouts in IC.

Often times, importing a sub-set is just fine - the reality is few accounts are completely "true," since most folks don't sign up with IC on day one of their catalog, and not all data is structured equally anyway. But in general it's better to fix issues within IC, rather than outside.