You can install your preferred R packages from CRAN where you can find over 9,000 (as of 2016/8/20) packages.
If you want to install R packages that are not on CRAN, then you can install them in the standard R console by setting ‘libPath’ so that the packages will be installed into the location Exploratory is referencing to, which is under /.exploratory/R .
.libPaths("~/.exploratory/R/3.4")install.packages("./h2o_184.108.40.206.tar.gz", repos = NULL)
If you want to use the functions from this custom R package as part of the command line then you want to add a custom R Script file that contains ‘library’ call so that your custom R package will be loaded. Please refer to this blog post for how to add R Script to the projects.
Here is a blog post that walk you through how to install and use your preferred R packages in Exploratory Desktop.