Sadly, if we need a custom voice line, it needs to replace another.. BUT, you can pick which one you want to replace! There is a tutorial on how in the zip file's "readme" file..
When playing some lambda fortress which uses the 2007 TF2 engine, I noticed he said something else than the usual thank you.
I saw none have done this so I decided to upload it!
I tried making the mouth movement work so much, its very hard and I gave up.