Well, one way is to extract your JavaScript out of your sin file and drop it right between the two sets of triple-single-quotes (much as I hinted at above)
'''
line 1;
line 2;
etc;
return mySin;
'''
That assumes there is a JavaScript var called mySin.
Now you wrap the triple-quoted code block in a call to executeJavaScript, something like…
String mySin = WebUI.executeJavaScript(
'''
line 1;
line 2;
etc;
return mySin;
''',
null)
Notice I’m not using WebElement - I don’t think that’s what you want… I think your SIN is a string of digits, right?
Now you can do…
WebUI.setText(findTestObject(...), mySin)
or whatever.