Most likely a simple REN command is all you need. I'm assuming you always want to preserve the original file extension.
ren "%original%" "?????????????????????-added.*"
Just make sure there are at least as many ?
as there are characters in the original name up until the .
Here are some results you can expect
original new
-------- ------------
blue.txt blue-added.txt
part1.part2.txt part1-added.part2.txt
You could use wildcards in your source file mask. The following would append "-added" to the base name of all .txt files:
ren *.txt ?????????????????????-added.*
See How does the Windows RENAME command interpret wildcards? if you want to understand why this works.