Paws Inc. doesn't own the rights to any characters created for the Garfield & Friends TV series. The production company (Film Roman?) owns them.
That's why Binky the Clown couldn't appear in the CG Garfield Show from a few years back. They found this weird workaround with Floyd Mouse, though. Since the generic mouse design that Floyd used originated from the strip, they just used the same design and the same voice actor (Greg Berger) but named him something different.
That's why Penelope can't appear and it'll have to be Arlene from the strip or a new OC.