You can do that if you use a Select File step.
In its Action tab, choose a local file or a file stored in a variable, and upload it.
▼Uploading from a local file
▼Uploading from a variable
Only the file fields are effective with the areas you can set in the Finders tab.
You can upload many files by combining this step with a For Each File step. For details about how to use For Each File, please refer to "How can I search files inside a directory with a "For Each File" step?".