Are you using ENVI or ENVI Classic? Is the original ROI file in .roi or .xml format? There is no way to import ascii files in the new ROI tool but if you want to edit an XML ROI, you can manually edit it since it is human readable and then again open it as an ROI. In ENVI classic you can create an ROI from an ascii file using the ROI Tool but it expects the data in columns of x, y.