I think it would be possible to take a few monsters from the Disciples game, for example: giant spider or Tiamat, yeti, ice giant, valkyrie, nightmare, avenger. To be honest, when I saw the expansion for the 5th part with gnomes, I hoped to see a yeti or storm lord with an ice giant there, but instead, there was some crappy tank.
I'll disappoint you: you can't create your own creatures or abilities.