Open DSP500.ini in Notepad. It's contained in the Windows directory. Should also be a link to to that in the Launch window's App pane.
Look for the section heading [devasync.dll].
Add a line for each port you want to add, in the form of COMxEnable=1, substituting the 'x' for the port number you want to enable.
The changes will be active the next time you run DSP.
Lastly...consider Do-more for your next app. Software is free and we've made a ton of improvements that eliminate the need to do stuff like this.