HubSpot provides integration with Salesforce.com to make it easy to track leads through both systems. A form submitted to HubSpot is tracked in HubSpot and the lead data submitted to Salesforce's systems as well.

However, Salesforce does not automatically store every piece of form data like HubSpot does -- it only retrieves form fields with pre-specified names. When you used HubSpot's form builder, you added the fields in Salesforce and they were automatically added to the HubSpot form builder. Now, using the leads API, things will be a bit more complicated.

Create the custom fields you wish to store in Salesforce's fields setup pages. (That's under Setup->App Setup->Customize->Leads->Fields.) Salesforce will provide you with an "API Name" for each field. When creating your HubSpot forms in Webforms, simply ensure the "Field Key" in Webform is the same as the "API Name" provided by Salesforce. Any field submitted to HubSpot which matches an API Name in Salesforce will be passed along with that lead to Salesforce; the rest will be ignored by Salesforce and only shown in HubSpot.

It may also be helpful to view the HubSpot documentation on this subject.