Its unlikely (OK - it WON'T) to work correctly (or at all) for several reasons (all Apache related)
A: Apache has a built-in system for this (multiple sites on a single server) - vhosts; and - (trust me) your 'webspace' on that host is in 'one of them'. One vhost can only have one DocumentRoot (webspace root)- PERIOD.
B: I won't get into the gritty details, but, on a server with shared IP's all running on port 80, the way Apache knows what vhost to send requests to is by the HostName i.e -www.yourlittledomain.tld - in the HTTP header request. Guess what? A vhost can only have ONE ServerName (HostName). - Period (no caps needed this time)
C: Even if you had access to the server httpd.conf, the vhosts wouldn't work right (as you are thinking of) as they would be nested - that would mean that the contents of the subdomains would also be part of the primary host, thus -- if they had different domain names -- they would have 'cross-domain' security problems (among many other issues).
Bingo - 3 strikes and you are soo, soo, soo out - you could just go home now...

Guess what else - when your Host provisioned the server - he/she/they/it already knew this, LOL.
Final analysis - Not going to happen -
But - a clever attempt

There are hosts out there that have VPS' around $10-15/month that will run YaBB (even multiples). You can apply your skills to one of those. Skills will be needed as it will be 'yo-yo' (unmanaged - you are on your own -- but you will become a whiz fast!) - Managed VPS's start around $30-40 month. Or you can get a hosting plan with support for multiple domains.
Edited: Afterward I realized something even more important - even if you could do this - you would unquestionably be breaking your Terms & Conditions with your host - which means they could shut you down at will, and (if adequately enraged) likely deny you access to your files. Plus -- think of the effect of this -- its exactly what hackers seek to do with hijacked domains -- little wonder it has been rendered impossible out of security concerns.
and - just a note on technique - Redirection points to another route, but it doesn't change the road signs (just like in the real world) and Rewrite only works within a server instance, both by design, for the reasons stated in the last paragraph.
Good Luck