I experimented with the function and command line, but no luck, thus far.
To rephrase, the proposed usage involves:
a) Selecting files to rename.
b) Pasting text from the clipboard as a prefix into the selected files.
*while adding to, but not replacing the existing file names.
The clipboard text should not conflict since each file has a unique name. Ideally, the rename string could be placed in a button 'function' field.
That should work as long as the clipboard contents don't contain anything invalid (like quote characters or other things that aren't allowed in file paths).