The activity sync is from Marketo to Salesforce, not the other way around so that will not help you with your challenge. In addition, I hope it makes sense to you that you cannot expect Marketo to read activities generated by Hubspot, as the format of activities will be different, assets and programs referenced will not be present in Marketo and Identifiers will be all different. That is why I suggested creating a custom activity. The main question however is what you want to do with all this information.
Usually, your want to know that a lead has gone through a specific campaign and what their end status in that campaign was. That is why I suggested to create some "legacy" programs in Marketo to match your important Hubspot campaigns and add the matching people (after your SFDC sync) to these programs, setting their status to what it needs to be. More about program membership and program statuses in Marketo you can find here: https://docs.marketo.com/display/public/DOCS/Understanding+Program+Membership
As mentioned, the key question is what do you want to do with the information. From the answer, you can figure out what the best way to make it available is in Marketo.
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