According to the documentation of the wso2greg , wso2greg provide different api types that enable us to interact with the registry , such as the registry-Api and the governance-Api, am already used the governance-Api to create some artifacts (wsdl ,xsd and serviceSoap with there dependancies) with java code and i already defined a taxonomy hierarchy via the WSO2 G-Reg Management , the next step is to select a taxonomy value(s) for an asset for example a soapService .
Am wondering if there's a way to do such a thing using the governance-Api.
As far as I know, requested operation is yet to implement in G-Reg. Which means WSO2 haven't implemented API to do taxonomy operations. You can always log an issue in GitHub on this. If not you can checkout the code from GitHub build your own API for this. Please refer this link if you want to build that.