You can export raw DN values from an image using the NUMWRIT algorithm. This is available from the algorithm librarian (Focus > Tools > Algorithm Librarian). In order to run this program with an ROI, you would need to ensure that your ROIs are in bitmap format. You can then set a bitmap mask as a NUMWRIT input. There are programs available to convert to bitmap, such as POLY2BIT and RAS2BIT.
You would also need to adjust the "Output Form" NUMWRIT parameter to specify the format in which image data will be stored in the output text file.