Our leads convert to contacts. (and the account is created) We create opportunities for each sale for that contact under the account. For our upsell campaigns additional sales are created as opportunities. Can I automate the creation of opportunities through Marketo? I know the opportunities sync, but can I actually create an opportunity as a trigger and use a default status?
Not directly. You could create a SFDC trigger that does it.
Another technique that is easier is to use and requires no development a feature call "Process Builder" that enables you to do this in SFDC.
There is only one way to do it, as a secondary effect of the Convert Lead flow step: However, as the article says: "converting a lead in Marketo will result in a new account and opportunity in Salesforce. If you do not want duplicate accounts, use Salesforce to convert."
Otherwise, Marketo does not write to Opportunities (or Accounts). I would solve this problem in SFDC, like Greg suggests.