How to configure expressions as Data Objects? How to resolve Data Objects dynamically (at runtime)?

I have a use case where I want to set the value of a data object (declared/defined in the BPMN), at runtime.

For instance, the BPMN should contain something like :

 <dataObject id="monthId" name="month" itemSubjectRef="xsd:string">

I want the variable “month” to be set as the current month in a particular format, which could be computed only dynamically.

How can I achieve this?
Is there a way to use a custom extension element? Or a way to convert/interpret this, using a class?(Like we can, for multiInstanceLoopCharacteristics, for flowable:collection, by using flowable:class as an extension element)

Note that my implementation is not based on Spring framework, and I do not have spring bean configurations, in my application.