If you put a container field on the same layout as your button, the button's script can use Insert File with the "store a reference option enabled". The user then finds the file they want, clicks "open" and the script inserts a reference to the file into the container field. This reference will include the file path and you can then extract the file path from the container field if so desired. (I use this method in "update" scripts that import data from the old copy of a file into the new. The user uses the dialog to find and open the copy of the original copy and then the script computes the file path and uses it with a series of Import Records steps to import the data from the old file into the new and to update any serial number field options.)
Worked great. Thanks.