Hi Brian,
SOAP is just a protocol that specifies how to transfer XML back and forth. So in a GeoEvent sense, you wouldn't want a SOAP input, per se, but an XML one. GeoEvent inputs (and outputs) are a combination of 1) a transport and 2) an adapter. Transports just know how to sling the raw bytes back and forth, whereas Adapters work with the data itself. At 10.3, GeoEvent has an inbound XML adapter. Out of the box, GeoEvent has two inputs that use the XML Adapter: "Poll an External Website for XML" and "Receive XML on a REST Endpoint".
I don't know exactly what you're trying to do but if you are receiving XML as a result of accessing a URL, then the Poll input may be what you want. It would continually reach out to that URL and process the inbound XML it receives each time. This is good for data that continually changes and receiving that data at a preset poll interval is sufficient. If you only want GeoEvent to process XML whenever it's truly necessary, whether at a high rate or low), then you may want to use the Receive input. You would then need to find a way to push data to the input's REST endpoint via HTTP POST, which is very easy using most programming/scripting languages. If you yourself are issuing the requests, i.e. via a browser, then you can download browser extensions (i.e. Chrome Poster) that send data the way you need.
Mark