If looking to inject custom content into a template at the time of sending, I would recommend having a look into creating a custom template that uses our template language.
If you've created a custom template within MailChimp using our template language to specify editable content areas: http://templates.mailchimp.com/getting-started/template-language/, then you would be able to update those content areas via the API.
To do this, you'll want to make either a campaigns/create call: https://apidocs.mailchimp.com/api/2.0/campaigns/create.php or a campaigns/update call: https://apidocs.mailchimp.com/api/2.0/campaigns/update.php and specify the section and content that you'd like to change as part of the 'content' parameter. the content 'sections' will correspond to the mc:edit tags that were added to the custom template.
You also have the ability to customize your content, like adding a first name to a greeting in the body of your content for instance, even further with the use of merge tags. I highly recommend checking those out as well and consider using them in your content if you need that level of customization: Getting Started with Merge Tags: http://kb.mailchimp.com/merge-tags/using/getting-started-with-merge-tags