I was also looking into making some tiny enhancements, and I also looked into the source code.
So, just for future reference: the source code explicitly says that the module is licensed under the GNU GPL (see http://www.fsf.org or google), that's an open source license which specifically grants anybody the right to make modifications to the source code, and even re-distribute it.
So, it's perfectly acceptable and legal to make modifications to the plugin or possibly even branch/fork it if you deem it necessary.
Personally, I would however suggest to get in touch with the developer (etruel)if you want to add new features - that'd surely be the best option to ensure that you're not stepping on anybody's feet (i.e. duplicate/redundant work).