Already got APB:R installed, want it officially installed on Steam? Follow this easy guide:
1. Travel to where you originally installed APB:R, default C:\Program Files (x86)\GamersFirst\APB Reloaded on a 64-bit machine (delete the (x86) if you are 32-bit).
2. Copy everything inside the folder, not the folder itself, but all the files within.
3. Begin downloading APB:R on Steam, pause, then exit Steam.
4. Go to C:\Program Files (x86)\Steam\steamapps\common\apb reloaded and then paste all your files in it.
5. Open Steam again (preferably run as administrator, just to be sure) and find your APB:R download, right click > properties and verify cache.
6. Steam will pick up all the files, download anything you're missing, and then you should be good to go. Uninstall the GamersFirst install of APB:R and delete any leftovers.
If you have any issues with this then please post.