Clarifying my previous post. There is room for ambiguity.
The activity data can not be easily exported from Hubspot and imported into Marketo.
The suggestion is essentially a separate database to store that data, allowing to use SOAP API to retrieve the activities and then relate to leads uploaded to Marketo.
A more intensive effort could restore a large portion of those Hubspot activities. There is no standard format.
The first step would be the movement of accounts, contacts and leads from Hubspot to Marketo. That is relatively standard.
The activities could be moved primarily through Munchkin JavaScript API, namely visitWebPage and and clickLink. That would allow to score, set interesting moments and set filters based on historical data. That is a partial migration of all activities.
The SOAP API would help with other data such as importing leads to lists.