Is there a way I can pass the data from Custom objects to Marketo fields? I have some specific fields on custom objects for which I want to pass the data.
Without the use of API's that's impossible.
Can you tell us more about your use case? May be we can propose a different way to solve the issue.
Recently, we have integrated On24 with Marketo. I wanted to pass on the data of what resources were viewed or how many times have a particular lead attended a webcast from the custom objects to the Marketo fields.
A solution, not involving APIs:
1. Use velocity to write the CO values to a plain email in xml formatting.
2a. DIY: Set up a server + inbox to receive the emails. Write server script to parse the email text as XML.
2b. Third party: There are paid services that can parse emails and execute webhooks, which would make things a hell of a lot easier.
3. POST the data to Marketo leadCapture endpoint w/ email address.
An increasingly common question, it seems. Are you trying to aggregate data across multiple records in the CO collection?
The only practical way to do this via a method I mention on my blog (won't link to the particular post here because the method is highly edgy and requires a developer, but you can find it easily via my profile).
As Greg mentions, you have to involve an API at some point. But he may be thinking of the Custom Object and Lead APIs being used but in fact you only need the Lead API and... well, the other part is the edgy part.