Creation date refers to the date of the lead's first activity -- including anonymous activity, from what I have seen.
This could be the same as the date the known record was created but I have noticed this being substantially earlier if the user was cookied and browsed around for days/weeks before converting.
Acquisition date is the date that the Acquisition Program value is set and should be the same as the date the known record was created (i.e., the date of conversion).
There are reasons this value could be blank though if the Acquisition Program is not being set properly. More info: https://community.marketo.com/MarketoArticle?id=kA050000000L547CAC
Since you are bringing in leads through the API, they will likely not have an Acquisition Program set automatically, so you should have a campaign that uses a flow step to set this value. The corresponding date should then be set automatically.
When you are uploading a list you can select the Acquisition Program and the date should be set.
From memory I don't believe you can set values for creation date, it is a system field -- but I could be mistaken on that point.