Hello
Hope this can help.
I think your question is about how to use UML instead of how to use the EA tool - if you understand UML 2.x, then what to do in Sparx EA will become obvious.
If I understand what you want to achieve, I suggest you read up on <<include>> and <<extend>> use cases. That will help you map out exactly what sequence diagram can be reused, under what circumstances (use cases are the preferred way to show what a sequence diagram does).
Then you can use combined fragments to implement <<include>> and <<extend>> in your sequence diagrams. "Ref" fragments can be used for <<include>> and "alt" or "opt" fragments can be used for <<extend>>. This will give you an accurate, precise and easy way to reuse sequence diagrams.
Sorry if I misunderstood the question! Cheers