Yes, you can send MIDI from another application to Pd. The actual method for doing this varies from platform to platform. See the paragraph MIDI Routing on the page midiinput for details on different operating systems.

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.