i have just a basic question: is it possible to implement an new projection with ArcInfo or FME? I want to use the rectangular pseudopolyconic projection by Györffy 2002, but is isnt listed in the standard-projections?
I don't know about FME, but not in ArcGIS. It is possible if you use the low-level projection engine library by implementing the algorithm yourself. You still wouldn't be able to reproject the data on-the-fly in ArcMap for instance or define a coordinate system for the data in the Györffy pseudopolyconic that ArcGIS could understand.
FME (CS-MAP) doesn't support the "rectangular pseudopolyconic projection by Györffy 2002" projection either.
You can write (or integrate) custom reprojection code and use it with FME. All your code needs to do is transform arrays of coordinates -- it is very simple. Then, you can use it in FME to reproject all of our geometric types: vector data (including true arcs), rasters, point clouds, etc.
This extension mechanism is very general and is not specifically tied to custom projections. To put this in context, users of FME normally use the CS-MAP or ArcGIS libraries to convert spatial data between coordinate systems. You can define custom coordinate systems in CS-MAP format (by specifying datum, ellipsoid, and/or coordinate system parameters), or in ArcGIS format (by specifying or selecting WKT for the coordinate system and/or geotransformation). There is currently no way to add a new projection type to CS-MAP or ArcGIS and have FME honor it. (At least no fast way: If you contribute your implementation to the CS-MAP open source project, or Esri implements it in ArcGIS, these would eventually be usable in FME too.)
If you are interested in extending FME with custom reprojection code, we ship samples and documentation with our SDK, an optional part of the FME Desktop installer.