I don't know if this is possible. It is just a bit of curiosity on my part, anyway. The Hero Lab character generator, which supports many game systems, has an option to export characters as XML files. Would it be possible to design a character sheet to import a character from this type of file. I know the RPGwithMe website (which is presently down for redesign) imported this way for their interactive character sheets. Barring that, would it be possible to reach out to the Hero Lab people at Lone Wolf to see if they could add a custom output for importing into Roll20. They do this for two stand-alone VTTs now, d20Pro & Fantasy Grounds.