If I'm being stationed in Europe, but I'm not airborne qualified (but want to be), how can I join the 173rd?

Unless you unit has a slot open for ABN school you have about two options. Go to Ranger school, pass and request to walk on to ARN school while still there (both schools are part of the Ranger Training BDE or when it come time to reenlist push your retention NCO to get you on assignment to 173rd with ABN School enroute.

I’m at Fort Hood, and i recently re-enlisted to go to another MI unit in Italy, mostly because...it’s Italy and my spouse is from there

give some more info where are you at now?? request airborne/ ranger school if in basic..
talk to your battalion command if at duty station, they will work with you if it fits the needs of the army.