The coalescense of the free space will happen either when you (as dba) tell it to or when SYSMON gets around to it. Depending on your system, it may not. However, when you allocate new objects in a tablespaces, DBWR or SYSMON is supposed to use contiguous free segments were possible, sort of coalescing.

If you imported too soon after your export, you probably did not give SYSMON enough time.

Have you though of using locally managed tablespaces with fixed initial and nexts? That should prevent the freespace fragmentation you are concerned about.