Can you add a custom object record from a custom activity? (In Marketo UI)

We track 'products purchased' using a custom activity. It would be very useful if we could add a new record to a custom object when custom activity is logged (if it meets certain criteria).

I had hoped it might be a flow step in smart campaigns, but can't see this when I tested it.

Any ideas for this, or can custom objects only be updated via the API? (we use a custom CRM).