Need to add sentences to Description metadata without deleting current description

hello! I am doing some work where I am going through a large archive and updating/editing the metadata. I've figured out how to do many of the things I need to quickly, but I can't solve my problem with editing the description metadata. Many of the photo files already have a description in them, which I would like to keep, but I need to add a sentence or two after the description that is already there (it might be a folder with 30 files, each with a different description that describes something specific about the photo, and I want to add the same sentences to all 30 files' descriptions without deleting the original, unique description in each file). It would save me a lot of time since right now I have to just click on each file separately and paste the extra sentences in manually. I thought the 'append metadata' feature was going to help, but it doesn't seem to work? When I create a template (with just the sentences I want to add in the description) and then select the files and 'append metadata', nothing happens. 

Is there a way to do this, or am I stuck entering this stuff manually? thanks in advance!