I have not done it myself but know people who have: it is possible to create bindings to a compiled C library to call from your Java code. A quick google serach of "using c in java" seems to bring up some possibly helpful results.
@Roth Indeed, this is possible using the Java Native Interface which allows a java to link to native code dynamically and vice versa. However, Max java objects already run inside a JVM created by Max. To communicate with Max externals you would need to send messages to symbols (see the SDK for details).
@ Roth @ Hans Höglund - thank you so much for the helpful tips.. In fact I am creating a database of math expressions in order to optimize non-linear situations, etc. I've coded mandelbrot, fibonacci, etc. I will code some more fractals, I will code some graphs, maybe some system for balancing of permutations (I don't know if you guys know something more specific about it).. and I want to pick up andré siers source code files in c++, from chaos-lib, in order, to include his externals in my database.. of course I will give him and the source code authors proper credit but there are actually a lot of work already done there.. and I would like to take advantage of it, once it is open source.. my object will probably be open source too