You could probably open up the speakers and clone the crossovers for the surrounds, but unless your center channel is at the same height as your fronts and also happens to sit vertically, it'll definitely be more difficult to design a matching center, assuming that the speaker was tuned as front/monitor speakers. You can, of course ask if the guy had designed a center channel and if he did, buy the crossover only
I'm personally avoiding a dive into full speaker design simply due to the massive time and effort involved in the design/simulate/test/repeat process of crossover/speaker design. Hurray for people that share their tested and proven designs!