Information on how to contribute to WEKA can be found in the Contributing a package section of the How are packages structured for the package management system? article. The conditions for new classifiers (schemes in general) are that, firstly, they have to be published in the proceedings of a renowned conference (e.g., ICML) or as an article of respected journal (e.g., Machine Learning) and, secondly, that they outperform other standard schemes (e.g., J48/C4.5).

But please bear in mind, that we don't have a lot of man power, i.e., being the WEKA maintainer is NOT a full-time position.