Hey Ju
Any suggestions are welcome - preferably opensource :)
I would think about something along these lines...
UDP Samplicator: http://freecode.com/projects/samplicator
Lancope Flow Replicator (commercial): http://www.lancope.com/products/stealthwatch-system/flowreplicator/
Flow Fanout (NetFlow only, but maybe simple to modify): http://www.splintered.net/sw/flow-tools/docs/flow-fanout.html
Or build your own solution with Elisa Jasinska Net::sFlow-Module: http://search.cpan.org/~elisa/Net-sFlow-0.04X/sFlow.pm