how to write a Classe in groovy script to reuse in several mock services scripts

May be you can create a separate project in preferred IDE of your choice using either java or groovy. Create compiled jar, place it under SOAPUI_HOME/bin/ext
Then in the groovy script step or mock script, instantiate your class and method.

Some one wanted how to debug the scripts, may be it is relevant for you too.
https://community.smartbear.com/t5/SoapUI-NG/debugging-groovy/m-p/118983#U118983