Help me understand what you are asking, just so I know. You want to customize the NOTE HEADS during the solfege lessons, correct? If you mean converting the actual huge words that flash on screen before the note is shown, I could be wrong but I don't know that there is a way to do that. However, to customize the note heads using your importing icon set,
-go to the lower right hand corner of the screen to Master Settings/Override and click the cogwheel to open it up
-Click the blue "Display and Sound" Option
-Go to "Icon" and make sure the radio button is green, not red
-Click the teeny tiny grey cogwheel next to the current icon setting to open up the icon options
-Click "Sets" and if it's already imported, you should be able to find it in the drop down list.
-At the top of that little window, select the option if you want the customized picture icon only to show or the icon AND note head. (You can come back in later and try it both ways to see what you prefer.
-Be certain to SAVE your changes, or they will not take effect.
-Before playing your lessons/files, make sure your override option is green and turned ON.
How'd, I do, clear as mud, right?
Let me know if this helped, if not, someone a bit more qualified can chime in to clarify as needed. Good luck!