The SoapUI plugin is part of the base package shipped with Squash-TA (it is automatically installed if you choose the default project configuration for your test project). However, as it's packaged as a separate plugin you can exclude it from the test project (and so avoid downloading and installing its dependencies).
This plugin includes all the necessary components to execute test cases from SoapUI workspaces as part of a Squash-TA test project.
To use a SoapUI workspace in your Squash-TA test project, you need to:
Copy the SoapUI workspace (XML) file under your 'src/squashTA/resources' directory.
Write a TA script under your 'src/squashTA/tests' directory in which you reference the SoapUI workspace file, convert it to the 'script.soapui' resource type and apply the command.
Note: Since 1.7, SoapUI plugin also works with bundles.