I don't see why not. I'm not sure if there's an easier way to do it all through the SDK libraries, but when you use a Retrieve Business Process Variables activity to retrieve file uploads from the Forms process they will be temporarily downloaded to a folder on the Workflow server's local drive.

Once you figure out the path (this will depend on the numbered folder assigned to the specific workflow) you can use .NET code (no SDK code required) to read the file into a stream so you can do the conversion. I don't have the links on hand, but you should be able to find at least one or two examples on here where people accessed the downloaded files using scripts.