Nils Petersen has completed a permanent transfer to Werder Bremen on a four-year contract for an undisclosed fee.
Ozkan Yildirim has signed a new contract to keep him at Werder Bremen until 2016.
Seaborne has won three and drawn one of the four matches he has played in since making the temporary move to The Valley.
The 25-year-old was forced to sit out the final match of his original loan spell, last Saturday's 2-2 draw against Brighton.
But Seaborne will get the chance to extend his unbeaten run in the Charlton team after a fresh deal was struck.