Yes, you can save the file as ISO. TO do this, firstly, load the he MDS file using File > Open. PowerISO can open MDS and MDF files without any other extra configuration. After you have opened the file and applied the necessary changes, go to File > Save as and choose ISO from the drop down menu.
Another solution is to convert the image directly to ISO. Open PowerISO, go to Tools > Convert. Input your MDS/MDF file, then choose ISO and the output directory and that's it. Now the file will be saved as ISO while keeping all its booting procedure (if any!).