Congratulations on your new toy ! What a great gift.
Yes, you can adapt an MP3 player to this system. In fact the Beosound 3200 you have just inherited can also serve as an amplifier for your television, DVD player, computer, and other devices.
The AUX socket TWG describes is located on the back of your BS 3200. Even if it is wall-mounted, you can still access it to add an MP3 connection cable. What ever sound source you have connected to the AUX will then be piped through your system to the Beolab 2000 speakers. The buttons on the BL 2000 will also allow you to access and select:
1. RADIO: AM/FM stations. Programmed stations can be accessed with the buttons.
2. CD: The BS 3200 CD player. You can scroll through CD tracks using the buttons.
3. STORED MEMORY MUSIC: You are no doubt aware that the BS 3200 includes a hard drive for up to 399 full CD's of stored music. All can be accessed using the buttons.
4. OTHER INTERNET SOURCES: Laptop, desktop, MAC, IPAD, etc.
5. TV or other sound sources via AUX.
Which leads me to ask two questions regarding your system:
a.) Does your BS 3200 system include a BEOPORT DEVICE ?
b.) Did your system include a BEO 4, Beo 5, or Beo 6 remote control ?
It would be helpful if you can take a few photos of your system, and whatever you can observe going in, and out of your wall cavity. You can upload the photos here on BEOWORLD, which might give more clarity and allow the members to offer better suggestions to assisting you with your needs.
Keep in mind that you should examine and understand what is actually going on behind the wall, and in the wall cavity regarding and connections, especially electrical power sources. You might want to augment the system, or make changes to better suit your needs than that of the previous owner. If your wall is common wallboard (Gypsum board,etc.), that can be done quite easily, and at minor cost.