Frankz advice will work as long as you get the .mbl file, which is where those custom tags are saved. See the backup & restore page on the wiki for more details.
In the future, I suggest making an actual custom tag that's saved in the file, so you don't run the risk of losing them of the files are separated from the database. However, transferring from "saved in the database" to a tag in the file is more involved than just switching the target in preferences. You'll need to create a second custom tag that is stored in the file, use the additional tagging add reporting tools plugin to copy the values across, and THEN you can change the original tag to point at the tag in the file.