mudassar,
There seems to be 2 objectives for your requirement.
1. To have the convenience of defaulting certain values.
2. To ascertain certain combination of Planner Group and Main. Work Center. i.e., Not to allow wrong combination.
The first one is not possible with easy means. Probably this space might not be able to suggest you the means of doing it. Sacrificing this might not be affecting your process, because this is a feature for user convenience. The second one is important, i.e., Fool-proofing the Planner Group vs Work Center data in the Maintenance Order can be done with the help of User-Exit IWO10009 with the help of a Z-table with fields Planner Group and Work Center. This Exit checks for correct combination then only allows an Order is Saved while Creation or Modification.
Even the idea of Z table will be redundant, if this combination is the same combination you have already maintained in the Technical Objects on which you are creating the Order (as rightly observed by Sebastian). If this is correct, you can use above exit to prevent changes in this defaulted combination. OR set these fields to Display mode through SPRO.
Another point is if your Order is Notification based, there is a user-exit for defaulting values QQMA0025. These defaulted values will be defaulted then to Order. and the story above begins.
So these are all the means. You only know the exact requirement of yours. Expecting the above information helps you find a solution.
Best of Luck
KJogeswaraRao