Yes, it is possible to develop custom features for Tangerine by accessing the source code on Github, which is linked here. The repository contains a README that explains in technical detail how custom features can be developed. There is also an “Issues” section to the repository where specific questions or concerns can be addressed and where you can suggest new features for development.
RTI is dedicated to making Tangerine as productive as it can be for its users. Please contact us with your features needs and we will be happy to discuss options for developing the software.