You are allowed to do this inside a Java action, but not a microflow.
In addition to rachana's answer, you may also use specializations in a microflow where the generalized object is the input parameter.
Nope, Mendix Doesnt allow such mappings. If you really need it. you can do a work around by:
create a dummy entity
have other entities associated to the dummy entity
pass dummy entity as parameter to your microflows.
It's possible with a little bit of Java magic, but I'd heavily advise against it.
Effectively you are asking for a solution that gives attributes a kind of global scope; their meaning would no longer be confined to the entity they belong to.
There are maybe a small number of usecases where this is fine, but in all other cases it would create a spiderweb of dependencies. Technical debt will take off like a rocket and development speed will slow down to a crawl.
Could you tell us more about your use case?