Could a founder be assimilated? I think not. If a Jem'Hadar were assimilated, would he still require Ketracel White? If so, Borg assimilation of the Dominion might be quite impossible. Perhaps a foundation for an unlikely "alliance". (of course "alliance" is in quotes because such a union would obviously be dictated by ulterior motives on both sides.) I'm not saying it's realistic. I'm saying "Use your imagination"