I recently updated to Unity 5.3.0f4 and I’ve quickly realized that I should have installed more modules than I did. However, there is no way to add modules via the Module Manager. So, I would like to know how to add playback engine modules (or any kind of module, really) to my Unity install.
I have a Personal license.

if i werent so poor i'd pay you. i didnt wanna uninstall and reinstall 13gb because my hdd fucked up, and no repair function.... didnt you could just overwite it with the installer....
– sunderplugs11To supplement: If you are on Mac and have changed the Unity application folder (for example so you can have multiple Unity versions installed), just rename back to /Applications/Unity before running Unity Download Assistant. That will save having to download the Editor again.
– Huacanacha