Well, yes and no. The koni inserts did not work in the stock RSX housings on the element, snowdreams tried and failed but he ended up using element struts/koni rsx inserts with eibach springs.
If your talking about taking a set of aftermarket full coilovers and not the spring on perch style, Emelent made a set of RSX KSports work on his but he said it was a lot of work. You will have to order custom springs rates, the shock body is shorter, the tie rod arm location is off so you have to use custom rsx shortened tie rod ends($300), and the rear strut bottom bolt has to be drilled out.
By time your done, your better off buying the teins or ksports(in production, to be realeased soon), because you will save money in the long run.
I personally dont recommend it unless you plan to not have the e on the road for a while, have a lot of experience with suspension components and good fab skills.
Good luck with your endevours.