Can European player sign for NBDL team during the season?
Yes, an overseas player can join the NBA DLeague during the season. If a player wants to go to the DLeague, he is placed in a player pool, and is able to be selected by a team via waivers.
Once the NBA D-League season begins, players who sign NBA D-League contracts are placed into a rotating waiver pool so that teams can claim them.
If a player from high school, college or overseas enters the NBA D-League without ever having declared for the NBA Draft, he will also join the waiver pool. That player remains NBA Draft-eligible but is not eligible to be called up to the NBA.