Taking a cue from Daniyar Kulmanov on possibility of using BRF+ with ChaRM, I tried the automatic Change Manager determination from BRF+ framework. It worked ! and here is the procedure to achieve it;
The standard SUPPORT_TEAM function is copied to Z namespace to adopt the Elements. You can also create your own Z function from scratch if you are familiar with BRF+. I chose to copy the standard;
Right Click > Copy
Name of target application should be 'SOLMAN'
Write down the Function ID of this newly created function from Details tab;
You can see the Signatures inherited from the SUPPORT_TEAM in the copied function (Z_CHANGEMAN_FUNCTION). It contains all relevant elements for the Service Desk.
We will create a 'Change Manager' element which is relevant in our case. To do this follow the procedure shown in screenshots;
Click on Add New data Object and provide the details shown below;
Now add the Change Manager in the Result Data Object replacing the Support Team. You may get following error when you try to Save;
To overcome this error, remove the Change Manager from the Signature screen using Remove Data Object button.
Now you need to create an Application, Ruleset, Decision table as you would create it for Support Team Determination. Instead of Component and Support Team in Decision table, use Category and Change Manager dataset; To configure application follow the steps mentioned in 5. CONFIGURATION IN THE BUSINESS RULE FRAMEWORK PLUS in the following link http://wiki.scn.sap.com/wiki/display/SAPITSM/Support+Team+Determination+via+Business+Rule+Framework+plus
Now we create a Z action in Z(Y)MCR_ACTION action profile for calling this function; this new action is similar to SMIN_STD_FIND_PARTNER_FDT action delivered in SMIN_STD action profile of Service Desk.
In the Processing Parameters provide below information;\
PARTNER_FCT : Partner Function of Change Manager
FUNCTION_ID : is the one which we created in BRF+
APPLICATION_ID: 0050568E6E9A02DE82A4EC0838CD95E5 (alwas constant, refers to Appln SOLMAN in BRF+
last step is to activate this action in Condition definitions. No need to provide any Start or Schedule condition for this action , just make it 'Green' in Conditions screen 
That's all ! now depending on your decision table entries, whenever you select a category in change Request screen, Change Manager is determined automatically upon Saving.
