Using this SDK may be most helpful if you have large amout of legacy code. As described in Create Your First Script | Mendix Documentation, Mendix SDK provides you an environment to create scripts to convert parts of apps.
There may be a large number of use cases where it may be best, to simply:
I don’t think there is an out of the box solution for such an conversion, especially considering you mentioning .NET. Legacy Java Apps (J2EE..) are so closely related to Mendix – as it also runs on a JVM as a Application Server – that an Developer should be able to handle the migration easily.
Mendix SDK for Scripting is well explained in Mendix Docs.
No, it's not possible to convert java project into mendix. Mendix project can be opened in Eclipse IDE to add custom java actions.