Partial File Rename

[ms_daniel]

This has probably been answered before, but I'm not sure if I saw the answer in computer speak I'd actually understand it. I'm an attorney playing IT guy here, and not doing it well.
I'm using BRU to rename thousands of pdfs I'm exporting from my database. The files have randomized names. I am using a spreadsheet along with BRU to rename them. No problem there. Now what I need to do is add a word in some, but not all the files.
Example:
From: 10015F10__cc216dfa_a62a_421c_8370_11b32340a45c.PDF
to: 10015F10_Heard_cc216dfa_a62a_421c_8370_11b32340a45c.PDF
I have the first part of the file name (10015F10) in a spreadsheet.
I want to use BRU to find each file where there is a matching first part of the file name and add that word in. I don't want to replace the second part of the file name and I don't have the second part of the file name in a spreadsheet. I'm not sure if I just made any sense, but basically I want to add a word to some files without replacing everything. Help?!

[ms_daniel]

This is how I've been renaming the files
Starting with version 2.2.6.0 it is possible to rename files according to a text-file. This text file should contain a list of "old name" and "new name" pairs, separated by a pipe symbol ( | ).
To use this facility, create a text file in for the correct format, for example:
Track001.mp3|Headlong.mp3
Track002.mp3|Rushes.mp3
TRACK003.mp3|AnywhereIs.mp3
As you see, there is one entry per line.
Next, click the Import option on the File menu and load your file. If the file is valid then you will see the "new name" column reflecting the contents of the text file.
When comparing the current filename with the text file the application ignores the case of the filename, e.g. it will happily match Track001.mp3 and TRACK001.mp3. This makes it easy to rename the files.
What I want to do using the example above would be
Track(insert word here)001.mp3
where I don't know 001