This is possible, but pointless. The kernel keeps a cache of data from the disk in RAM. The data that you used most recently is kept in RAM. You will naturally end up with the parts of /usr that you use often in RAM, and the parts you don't use won't be taking up any RAM.

If you want better reaction time when you start an application, you can seed the cache. A file is loaded into memory the first time you use it, and remains there until the memory is reused for something else. You can force a file to be loaded:

cat /path/to/file >/dev/null

For example, to preload all executables and libraries into RAM:

cat /bin/* /lib/* /usr/bin/* /usr/lib/* >/dev/null

This can take a while to complete, so you should do it in the background. You can put the following command in /etc/rc.local:

ionice -c 3 cat /bin/* /lib/* /usr/bin/* /usr/lib/* >/dev/null &

To also load all libraries in subdirectories of /usr/lib* it could be useful to run find:

ionice -c 3 find /bin /usr/bin /usr/lib* -type f -exec ionice -c 3 cat '{}' ';' > /dev/null &