I am sure this is not supported and therefore not recommended but installing software in the vSphere Management Assistant is possible as it is just a SUSE Linux Enterprise Server 11 virtual machine. You can use the following to search for software:
sudo zypper search nano
This will search for nano. Once you have found the package you want to install then use the following to install it:
sudo zypper install nano
Note the first time you run this you are prompted to accept the openSUSE package signing key. This is up to you and no one else...