I 2nd the advise of @brolly33 with packaging up the contents of the network share using something like the Software Distribution Wizard. This will ensure you know EXACTLY what will be deployed to the endpoint when you take an action.
The BES Client runs under the Local System account on Windows which (typically) cannot access network shares. You would have make the network share accessible to non-authenticated guests (NOT recommended) or in your actionscript call a command to perform the copy as the currently logged in user (requires that any user this could run as has access to the network share). While there is a use case for this, it does not leverage the native data delivery capabilities of the BigFix architecture nor does it provide safeguards around data integrity.