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I have installed Raspbian with NOOBS and I now want to add another operating system (OpenElec) alongside it. When I hold Shift to enter recovery, I only get the option to install Raspbian again.
How can I add another OS without wiping what's already on the SD card?
Don't waste your time, that's not what NOOBS has been designed for. NOOBS has been designed to make it easy to select and install operating systems for the Pi without having to worry about manually imaging your SD card. There is no hidden option to add an OS to the already installed OSes.
My suggestion is to make a backup of your data, reboot into the Recovery screen, add Raspbian and LibreElec (OpenElec) to the list of OSes to install and install everything from scratch. Finally, bring back your data. Except you are a Linux expert, this will be the best way to go without wasting time.
Reinstalling software is usually not a big task. I need less than an hour to reinstall and configure a complete development and test environment on my Raspberry Pi 2 inclusive SSH, C#/Mono, Go (golang), LiteIDE, MySQL and more.
Personally I'm using a FileZilla client on my Windows PC to backup my data from Raspian over sftp/ssh. It's as simple as to enable ssh in raspi-config and then connect to the Pi using FileZilla with sftp and my Pi user credentials.
Regarding your question, I don't say it's impossible, but it requires a deep knowledge of NOOBS, its internal configuration and config files as well as the Raspberry Pi's partitioning and boot process.
It starts with the problem that your installed OS already uses the entire free space on your SD card. You would have to change partition sizes, extract partitions from OS images, arrange them correctly on the SD card and finally manually and correctly edit NOOBS' JSON config files.
It's certainly not something that can be explained here in all its details, nor something that can be done in a little while without having a broad and deep knowledge of any aspects involved.
However, in case you have a good knowledge of tools like fdisk, parted, mount (to extract images with offsets), rsync, mkfs etc. and you are willing to dig into the details of NOOBS, I suggest to start with the NOOBS documentation on Github and the pages in the NOOBS wiki.
Using spare SD cards to install everything you want is probably the fastest way to get something usable, ...but... It's not the only way to do that.
Noobs can be used to install other OSes/images (no matter if they're prebuilt from Foundation) and it's easy to have a multiboot system on the same SD card. Noobs is not just for noobs, things are changing fast from its first release.
I have always skipped Noobs because when you have a development Pi on your desk it's better to have multiple SD cards and manually switch between them, I have reconsidered it a lot after my latest home media center.
After OpenElec (primary usage) I wanted to have RetroPie (for spare time) as well as a full blown raspbian (Chromium, occasionally browsing) in the same machine. This Pi is on the back of my LCD TV in my living room and having a multiboot system is really useful (and fast). Switch between these OSes might be done easily (openelec/raspbian/retropie plugins, your scripts, hw keypresses or ...) and you've a suitable system in a matter of minutes. Here are few references:
But remember: Noobs is not tied to ready made or default OSes from RPi Foundation, you may safely create and use your SD images easily
