I don't know if this helps or not, but I found this on page 365 of the ST 11.20 user guide regarding external data sources:
1 In the Data Window, select the parent table in the Data Explorer tree.
2 In the canvas, select the Test Flow or loop frame.
3 Click in the Property Sheet to open the Data Sources view.
4 Click Add. The Attach Data Source to Loop dialog box opens.
5 Select a data source from the list and click OK.
6 To set a navigation policy, double-click the data source or click Editin the
Property Sheet.
Tip: The data source is available only tosteps within the current loop. To
allow access to the data source from all steps, add it to the parent loop,
such as Test Flow.
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There is also this on page 133 of the ST 11.20 user guide:
a Make sure the Service Test test you want to call has been saved and run
successfully at least once.
b Expand the Unified Functional Testing node in the Toolbox palette.
Drag the Call Service Test test activity into the canvas.
c Open the Property Sheet's Generalview and click the Select ST Test
button. Select a test version 11.00 (or higher).
d To update a test that has already been loaded, click Update ST Test.
e To specify a custom directory for the results, click the Browsebutton
in the Results directoryrow in the Generalview tab.
f Open the Property Sheet's Input/Checkpoints view and set the
property values. The property list remains empty until you select a test
as described in step c. For information about selecting an external data
source, see ''How to Assign Datato Test Steps'' on page 365.
g Add other relevant steps to your test. You can link subsequent input
properties to the output properties of the Service Test test step. Click
the Link to source button in the property's row to create the link. For
details, see ''Select Link Source Dialog Box'' on page 395.
h The Service Test test step input must be a string, so if you want to use
the result of a previous step that created XML, add the XML to String
activity. For details, see ''XML Activities'' on page 187.
i Save and run the test.