If sorbet is not firm enough after freezing, can it be returned to the icecream maker for further churning and refreezing?

You can try to rechurn, but first you need to determine why it isn't firm enough. One of the critical factors playing into firmness of a sorbet is the sugar balance, although if your recipe includes alcohol that can also play into it. I suggest that you read up on the subject at https://www.seriouseats.com/how-to-make-the-best-sorbet
That's the best place to figure out things regarding sorbet, if you ask me. And yes, if your machine is still cold enough to process the sorbet mixture, you can return it to the machine to continue the process.