It is possible to import existing transcripts in the zip file upload.
First, create a folder with image files in it. Then make sure that each image file has a .txt file containing the transcript of that page, following the same name conventions as the image, like in this example:
Not all image files need corresponding text files, but the filenames do need to be identical (except for the extension) when there are text files with transcripts.
Create a metadata.yml file if you wish, and place it in the same folder.
Then zip up the folder (along with other folders, if you want), and upload it to the Start a Project screen. Make sure to check the "OCR" box.
The folders should be converted into a FromThePage work with the contents of the text files set as the raw OCR text. You'll probably want to convert the work into a manuscript transcription work from an OCR correction project so that the nomenclature is changed appropriately.