In addition to Chris Johns' answer I have done further research to answer this question and have discovered the following:
(1) In general, there are no special customizable motors made; motors are made in such a way that they can be taken apart with the right tools, so every motor more or less is customizable with the right tools.
(2) Some manufacturers sell "modifiable" motors that allow a buyer to alter the way the motor is configured to a limited degree, but usually you must be making a bulk order to take advantage of this. Note that such "modifiable" motors only allow the customer to switch out components for fixed alternatives that the motor manufacturer has pre-determined.
(3) To change the bearings on a motor requires equipment such as bearing pullers and snap ring pliers. It may also require an arbor press for the larger motors or even a hydraulic press for really big motors.
(4) In many cases you can get different bearings with the same form factor, so you can change your bearings to a different type with no further modification. For example, you could change ball bearings with roller bearings.
(5) If your wanted bearing is in a different form factor, you may need to modify or fabricate the plate that holds the bearing to the frame, or alternatively create an adapter that fits to the existing plate.
(6) If the motor does not have enough room to accommodate your bearing, you can remove the motor's bearings entirely and place your bearings outside the frame. This will probably require removing the motor's bells, the end caps. Your subframes for holding your custom bearings must be aligned so that the spacing between the rotor and stator is exactly equal all around. (If you can improve this spacing, you may be able to improve the performance of the motor over the factory build.) You will probably need a new, longer shaft to place bearings external to the existing frame.
(7) The shaft of nearly all motors is joined to the rotor with a simple interference fit, or a force fit in the case of very small motors. To remove the shaft warm the rotor to approximately 200F using a heating strip or other mechanism and cool the shaft using freeze spray, a cooling jacket, dry ice, or some similar mechanism. The shaft should now be removable with an arbor press. You can try to force the shaft out cold with a 10 ton hydraulic press, but this risks damaging the rotor, so using a thermal differential is recommended. A new shaft can be inserted into the motor by the same method.