I would expect that to create a REST project, TestSuite and TestCase from a WADL is certainly possible using Groovy Script, but before going any further, given that you say this is based on a URI/WADL that the user provides, may I ask:
a) Why you want to do this and why the usr cannot just use the menu driven functionality e.g. new REST project from WADL?
b) Is it that you want to generate the project on the fly and have many different WADLs to use? If you just want to vary the REST service endpoint / request values of the tests, then there are easier ways to achieve this.
for a) I take it you know how to get the GUI to generate a REST project / TestSuite / TestCase based on the WADL/URI? e.g. File New REST Project > Import WSDL (or use URI) etc and generate TestSuite, TestCase and REST Test Request Test Step? Sorry if this is not what you meant.
for b) You would need to parameterise (using property expansions) the enpoint / request params for the REST Test Request TestStep - to do this, have you seen this other questsion & answer that came up today it covers this topic: