However, you’d have to manually add and configure:
-the emulator config folder
-the ROM folder
-the EmulationStation config (this involves editing es_systems.cfg to include a section for your new emulator)
First, though, try using RetroPie-Setup and see if the emulator you want is available. It may be that the developer has already set this up but hasn’t added it to the main image. Try the experimental section.
Which emulator are you trying to install, do you have a version that runs on a Raspberry Pi and what kind of RetroPie are you using (did you install using the script, the SD image, did you update anything to the latest version yet?)