As well as, or instead of, setting PATH environment variables in startup files, you can simply add a path as a file to the directory /etc/paths.d
For example, here is a command that sets up a path to a custom installation of MongoDB
$ sudo sh -c 'echo "/Users/jones/Documents/mypath/mongodb/bin" > /etc/paths.d/mongodb'
Start up a new shell and you will find that path added to your PATH environment variable.
Note that this is a system wide setting and so it gets set early in the process of shell creation. If you need to override PATH settings for specific users then you still need to set paths in .bashrc, etc. I don't know the order in which /etc/path.d files are sourced but I would assume alphabetical.
Also note that you need the 'sh -c ' construct in order to create the file as shown above. Simple 'sudo echo' will not work.