Here is a brief tutorial to modify and add existing "http.uri.params" in Mule. Below is the code for reference:

1) Create Java Transformer in Mule.

2) Associate with any Java Class which extend AbstractMessageTransformer class.

3) Override "transformMessage(MuleMessage message, String outputEncoding)" in that class.

4) Use below to do the modification/addition:

ParameterMap map = message.getInboundProperty("http.uri.params");

For example, if you want to override/update/add email param in uri. Use the below URI:

map.put("email", "rohit061989@gmail.com");
message.setProperty("http.uri.params", map, PropertyScope.INBOUND);
return message;

5) Use below code to set new uri param in case "getInboundProperty("http.uri.params")" is null:

ParameterMap map = new ParameterMap();
map.put("email", "rohit061989@gmail.com");
message.setProperty("http.uri.params", map, PropertyScope.INBOUND);
return message;