My experience has been with both welding and pieces similar to the one described below. Welding is sometimes difficult, even following the procedures.
But if for some reason you cannot weld, sometimes you can bolt something. I dont know your geometry or loads, but using a milling machine you could machine a centered hole, and a face on the gearbox case. Then on a lathe, you could manufacture a flange that will be fixed to the machined face with bolts; this flange can be easily machine to receive your bearing. The flange will center with a lip on the hole. To apply the bolts you can thread the case itself, or use some nuts on the interior.
Dissasembly procedure would be slightly modified, since it is possible that you would have to remove the flange to get the bearing or the shaft.
sancat