Oh. No, i don't think it's possible - i'd suggest to just use multiple subdomains.
Technically, you can squeeze out some bits: there are 36-37 possible characters of which you are using only 32, so with arithmetic coding you would be looking at about 1 extra bit for 5 characters, but it's a nightmare to code. And after those extra bits run out, you will get the same problem anyway.
Technically, you can squeeze out some bits: there are 36-37 possible characters of which you are using only 32, so with arithmetic coding you would be looking at about 1 extra bit for 5 characters, but it's a nightmare to code. And after those extra bits run out, you will get the same problem anyway.