I think you'll need either a system script or a file manipulation plug in for that. There are several plug ins that can list the contents of a folder--which could then be parsed for any files ending in *.ans.
On the other hand, if you were to locate that directory in the documents folder, there's a get function that can be used to list the contents of that folder...
I almost forgot that the filename will be specific this time as it will depend on the user at the time.
ie. \\aci-2008-01\Shared\Pro73\xy\hcd.ans where hcd is the get (AccountName). Since this will not be in a documents folder, I'm curious what system script can work to accomplish this. Since this is intended to be an XML import file coming from an external program, I wonder if something to the effect of attempting to perform the import a certain number of times (up to 50 attempts) would do the trick.
System scripts depend on the operating system. If you have a Mac, you might be able to create an apple script that takes the value from a field where you store the result of Get ( AccountName). If you are a windows user, you might be able to construct the text needed for a batch file in a Filemaker text field--which gives you the ability to insert the current user's account name. Then you can use Eport Field Contents to export the text to create the batch file. Send Event can then open and run the batch file. You'd be creating a text file listing the directory contents with this batch file in a known location such as temporary items and then you'd import that text to get a list of the directory contents.
All in all, a plug in seems simpler, but I recognize the logic behind not using a plug in if it can be avoided. (I believe MooPlug lists directory contents on windows machines and is free...)