Yes,
But, Workflow is a better option if you need to send email, change a entity fields create new records when something is change and so one.
Custom WorkFlow Activity, is like a plugin, is custom code in c# or VB.NET using CRM SDK. Inside you can do that operation with no problem (remeber that if you are in CRM Online you have SandBox environment so acess to other databases can be painful (https
need));
if you want to integrate 2 systems, use plugin as asynchronous, is a better choise that CWA.
kind regards