There's a process for it. I'm not sure it costs anything but it's a bunch of paperwork. You have to justify what it's used for, why existing solutions don't work, etc. The working group is probably pretty reasonable, but I'm sure it's an involved process.
If you do, can you please tack on symbols for following external links, space bar symbol, and all the other miscellaneous internet adjacent characters I always have to reach to Fontawesome for?
A unicode symbol? I want a symbol! How much are we talking about?