If I understand your question correctly, it sounds like you want to use a rate rule. If you want to build a rate rule in the diagram, take a look at this table.
The above answer assumes you don't want to use a reaction based on seeing "without reaction rate" in the question summary. But maybe I should try to understand whether that's really a requirement. Because it's also possible to express the equivalent rate rule as a reaction. For example, you could creating a reaction "null -> X" with reaction rate set to "E" would be equivlalent to the rate rule "x = E", which is equivalent to the differential equation "dx/dt = E".
