MathWorks defines MIMO transfer functions as either a concatenation of SISO transfer functions, or a cell array of SISO transfer functions, as mentioned in the documentation.
Please run the below command in the command window of installed MATLAB R2019a version to get release specific documentation
>> web(fullfile(docroot, 'control/ug/mimo-transfer-function-models.html'))
Assuming you have the input and output for each pair of input and output ports for this system, you can create new transfer functions with "tfest" using the old transfer function as an initialization point. The "init_sys" input parameter for the "tfest" function will accept idtf models, such as existing transfer functions.
Please run the below command in the command window of installed MATLAB R2019a version to get release specific documentation about "tfest":
>> web(fullfile(docroot, 'ident/ref/tfest.html'))
You can create a loop to update each SISO transfer function individually using this method to update your previously trained "tfest" model with new training data for offline estimation.
Please follow the below link to search for the required information regarding the current release:
