A Mellotron is basically just a sampler that used a selection of recorded material. A good bit of keyboards don't actually use synthesis for sound production, but instead rely on samples of actual recorded material. So assuming that you can find audio samples recorded from a Mellotron, you could possibly load them into a sound bank of any sample-based keyboard and effectively emulate the concept, although it wouldn't be as cool as actually having a Mellotron.
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