OK, that is clearer. First, you don’t need an outside program to USE fonts in PSP. In fact, PSP will use the fonts from your Windows system. One option is to install new fonts, but this can easily get out of hand as it is likely that you will find more and more fonts, and those will also end up available in other Windows applications, like Words, and such.
If you are using the old PSP7, I suspect you are not using Windows 10. Are you using Windows 7 or still use an old Windows Xp?
If you happen to use Windows XP, you could simply double-click on any font you might have downloaded and want to use, and that would open a display of that font. As long as that display is open (even if minimized at the bottom of the taskbar), that font will be available in PSP.
If you are using Windows 7, that function has been removed and you would need a font viewer, like TheFontThing. There is an article on font viewers HERE. If you use The Font Thing, as long as the font is VISIBLE in the right pane, it will be available in PSP. As soon as you close TFT or you scroll further to display other fonts, that one you wanted is no longer available. Of course, you can temporarily install a font and uninstall it once your project is done.
Let me know if this helps.