I am agree with MarcLasserre_GCS and KnowHowPRPC, call an activity from a data transform is not recommended.

If you are really need to call an activity, you can use the method doActivity available in the engine API (available with Resource -> Engine API in designer studio).

To use it, you create a function. In this function, you create :

• A stringMaps with the value "pxObjClass" (equal "Rule-Obj-Activity"), "pyClassName" (equal apply class of the activity) and "pyActivityName" (name of the activity)
• A parameter page with all necessary parameter for the activity (can be null to use current clipboard page)
• A clipboard page if you want run the activity on specific clipboard page (can be nullto use current primary page)

Next, you call the doActivity method with previous created object in parameter.

For example if you want to call activity @baseclass.TestActivity with parameter "param1"="Test" and current primary page then the corresponding code is :

// Create the parameter page
ParameterPage newParamsPage = new ParameterPage();
newParamsPage.putString("param1" , "Test");

// Set activity name
HashStringMap params = new HashStringMap();
params.putString("pxObjClass", "Rule-Obj-Activity");
params.putString("pyClassName", "@baseclass");
params.putString("pyActivityName", "GetListOfFieldValues");

/Execute the activity
tools.doActivity(params, null, newParamsPage); 

After, you can call this function in data transform.

You can find a lot of example in OOTB function.