I looked at the online documentation for LMMS. Under the section "Exporting The Song", there is a discussion regarding file format, sample rate, bit rate, bit depth, etc., but nothing regarding entering additional metadata other than naming the song. Being a free download, my guess is they have elected not to add the features found in professional DAW software. So you may be out of luck with regards to editing any metadata fields.
ProTools is quite expensive. I just mentioned it above because that is what I use in my studio. Perhaps the OP should download the free version of ProTools First and import the finished audio into that, and then export the clip as a wav or mp3 file, adding the metadata there. On Avid's website, there is a "system requirements" section for ProTools First to see if your computer is up to the task. From what I understand ProTools First is just a stripped down version of the latest full featured version of Pro Tools.