Well I don't think you have defined your requirements well enough to say it can be done or not on a arduino. However a arduino mega board does have four serial ports available so under certain conditions serial conversion is certainly possible. But again requirements have to be more clear, such as:
Is this to be a one way conversion, that is there to be one input stream to the arduino converted to one or more streams outputting at other baud rates from the arduino?
Is the one input stream the slowest of the baud rates relative to the output streams? If not then possible additional buffering might be required in your sketch, above what the arduino hardware serial library makes available.
Lefty