IIRC, the Dutch were religiously tolerant towards anyone not a Catholic, and Jews did fairly well under the Republic. So I can imagine the Republic allowing the Muslims in the Spanish Netherlands to cross over, revert openly to Islam, and form military units to fight their Hapsburg oppressors, in exchange agreeing to swear loyalty to the Republic and not try to convert any Christians (Calvinist or otherwise).
Eventually, some of the more well off ones would engage in trade-Islam, like Calvinism, is a very mercantile oriented religion and the Muslims will have connections in North Africa and the Middle East that would benefit the Dutch. So I imagine you'd get a small, but influential, segment of wealthy Muslim businessmen in the Republic, and a larger population of farmers and soldiers, some of whom may immigrate to places like Brandenburg after the Thirty Years War if the rulers offer them a good enough deal.
I can't help but thinking how would this influence Dutch activities in Indonesia later on. Inevitably some of them will be invented in VOC or anything equivalent....