If I'm understanding correctly, you should be able to include the brs file that has your methods by including it in the task's xml file;
<script type="text/brightscript" uri="pkg:/components/measure.brs" />
And then your task would have access to those methods.
There's also the possibility to create objects with functions defined within and pass the whole object in, not sure if it'll work with tasks this way, can't recall doing this yet with a task... but don't see any reason it shouldn't work. Here's the general form;
function parentFunct(params = whatever) as object
this = {}
this.myChildFunct = function()
someResult = DoStuff()
return someResult
end function
return this
end function
aspiring