The SBS version of Backup Exec, just like the SBS version of Windows Server, is designed purely for a standalone server environment, where you have one server in the office which does everything, therefore as far as I'm aware it doesn't support installation of remote agents for backing up remote servers.
My guess would be that you can't install any agents into an SBS installation, however, since from looking at prices I've got here I can see that a copy of the SQL Agent costs more than an entire copy of SBS Premium, if you can't get your copy changed from Standard to Premium, it would be cheaper for you to just ditch the current copy and buy a Premium copy, than it would be to buy a SQL agent in the hope that it will install.