Crucible native repository access says:
Crucible natively supports Subversion, CVS, Perforce, Git and Mercurial repositories.
We have third-party SCM plugin which is of type lightSCM plugin.
1. How to convert 3rd party light scm plugin that would list under native repository access like Git.
2. What is the procedure to do so?
3. What is a proper channel to get the way for the native plugin development?
You say you have a plugin already, so I'm not sure what you are thinking you actually need here.
Assuming that statement is wrong and you do want to create a plugin for lightscm as the second and third questions suggest, start at https://developer.atlassian.com/