Is it possible
to attach ( with a proper connector ) an "isolated"
midi device directly to the soundblaster midi joystick iinterface?
I don't think so; one is a voltage-level interface, the other is more
like current-loop. (ICBW, of course.)
However, if "with a proper connector" is taken sufficiently
liberally, the answer is "yes"; there exist interfaces that are built
into DA15 shells - you plug the DA15M it into your joystick port, and
it breaks things out into three connectors: a DA15F for the joystick
(if any) and two MIDI connectors for, well, MIDI. Mine cost me CAD
34.45, TTC, around the middle of last August.
There's tons of these things on ebay for US$15ish + shipping each.