Yes, I can substitute St Germain for Lillet. St Germain is, in fact, a liqueur in itself. And it is, in fact, an eau de vie (double distilled grape brandy) like Lillet.  You can use St. Germain in place of Lillet Blanc; although it is also quite a bit more expensive, it will work just fine.  You can also think about other modifiers, such as orange or raspberry liqueur or even fruit-flavored vodka.