You can use erplab to export as universal .txt file and then change extension to .ep by hand
1. Load erp into erplab
2. In erplab, export erp to text universal (time = unchecked; include electrode labels = unchecked; transpose = points)
3. Once it's exported, you should have a file with rows as time points, columns as electrodes, and no header information for both rows and columns. Now you can just change extension from .txt to ep. by renaming the file (right click with mouse, rename, change extension). Windows will ask if it's okay, say yes. Done.
Maybe there's an easier way, but this is how I do it and the file opens in Cartool
Hope this helps