Why don't you try in your system...
Press F4, Can you see the movement type 311 with special stock M ?
As you can see this is not possible to use special stock M for movement type 311...
So you can explain your business process.
SAP has given solution for all types of business process, if any standard process doesn't match with your requirement, then you have to go for a development...
But before that, please explain your business process for the requirement for special stock M for movement type 311 in MB21 ..