Can Shapeways electroform your plastic 3D printouts? No, they cannot. However, there are companies that specialize in doing this for 3D prints, such as this one called RePliForm. First they electroform with copper, then they electroplate with nickel, that is their standard process. And I agree, it is a quite versatile one at that!
20200930/DuckDuckGo shapeways electroplating
20200930/https://www.shapeways.com/forum/t/can-shapeways-offer-us-electroplating.917/
20200930/https://www.shapeways.com/forum/t/metal-coating-wsf-models.5445/
20200930/http://www.repliforminc.com/
WSF (White Strong + Flexible) is the name of the preferred Shapeways material. But what is it really? It is Polyamide 12, so yes, Nylon.
20200930/DuckDuckGo shapeways wsf material
20200930/https://www.shapeways.com/forum/t/wsf-material-maximum-size.96204/
20200930/DuckDuckGo shapeways white strong flexible
20200930/https://www.shapeways.com/blog/archives/97-glueing-white-strong-flexiblesls.html