Yes, you can send MIDI from another application to Pd. The actual method for doing this varies from platform to platform. On win32 platforms, you will need a special "loopback" program. On GNU/Linux platforms, you need to use the ALSA virmidi drivers and configure them correctly.

You are also not restricted to OSC in that you can use [netsend] and [netreceive] to move data between other applications and Pd, or multiple sessions of Pd on different nodes of a LAN.

Also, if you use the [netreceive] object, you can send messages to Pd through a network socket from any program.