You're confusing many things here. First "Buildroot Linaro" doesn't mean anything. Buildroot (http://buildroot.org) is an open-source project that provides an embedded Linux build system, Linaro (http://linaro.org) is a non-profit organization that enhances the support of the ARM architecture in the Linux eco-system. The main interaction between the two is that Buildroot can use a Linaro toolchain as the cross-compiler.
Indeed, there is nothing like "aptitude" on a Buildroot generated system, because we don't support binary packages. See http://buildroot.org/downloads/manual/manual.html#faq-no-binary-packages for details.
Finally, you're confusing Buildroot and Busybox. Busybox is a userspace application that provides a lightweight implementation of many basic Linux utilities. It is one of the 1600+ userspace packages provided by Buildroot.
Busybox definitely doesn't provide a Bittorent client. However, Buildroot indeed has a package for transmission.
So if you want to move forward, you need to get the source of the Buildroot that WD used to generate the firmware to your device, modify this Buildroot to add transmission, then re-build the filesystem using Buildrot, and finally flash it on your device.