Your question is kinda unclear. I'm assuming you mean you want to set up a pendrive with more than one freebsd installer img/iso on it, and the ability to boot off the pendrive and choose which installer to run?
The only way to do this is to set up a miniature pre-boot loader. In the linux world the SYSLINUX suite is used for this, but IIRC it used to have problems booting bsd-based systems. I'm not sure if that's still the case these days, but either way it's probably your best bet.