Mint.
MIDI is dead easy to generate, it's just serial 8 data bits no parity and 1 stop bit at 31,250 baud.
To send MIDI from buttons all you'll need is a microcontroller, almost any type will do, the buttons, a couple of resistors and a power source for 5V or so at about 25 mA.
If you already have a USB-MIDI interface and you're not using the output from this interface it's often possible to power your circuit from the midi out or thru connectors.
In the past I've used PIC12C508 chips and some very simple code to generate control codes and note codes from buttons or foot switches, but it really depends on how many buttons you want.
If you can give us some more info on what you need I/we can probably help more.
Do you have any preference of microcontroller?
Do you have any programming equipment?
It can be done with just logic chips if you only want to send a couple of codes and don't have any microcontroller stuff.
Brenden