Welcome aboard Markus,
Here is a link to a 1994 2.3L 940 Volvo TB
940 1994: Throttle Body Service
If it is the same that you have, the larger hose on the top may be the Positive Crankcase Vent., you'll have a better idea when you jump into it. Pic's before hand would be a good thing.
Unfortunately he didn't have a photo of the bottom.
He actually talks you through the removal so that's a plus.
Bit size would be judged by the wall thickness of the TB, to break through in a couple of spots is no big deal but any more than that and you can risk a fracture in the TB over time.
If you have JB Qwik there in Sweden or an equivalent you can add some to the outside of the potential holes before you cut the groove.
as soon as you have pictures uploaded it would be easier to make a better determination.
If you haven't watched any of Ron's video's you might want to check them out on Youtube
Search for Gadgetmangroove , gadgetmanglobal or GroovyServiBulletins
Lots of good info in the video's.
Can't wait to hear/see the results
Karl Fortner
Tacoma, Wa.
Gadgetman Tacoma