Hi Felix ,
I may be too late to answer because i was searching for same answer and i got the solution and i have implemented the same , so below is the answer,
In webdynpro view ( Where you have your table and its popin ) ,
please write the below code in your WDDOMODIFY view ,
DATA wd_table TYPE REF TO cl_wd_table.
DATA wd_table1 TYPE REF TO cl_wd_table.
CHECK first_time is NOT INITIAL.
wd_table ?= view->get_element( 'TAB_SEARCH' ).
WD_THIS->TABLE_METHOD_HANDLE ?= wd_table->_METHOD_HANDLER.
WD_THIS->TABLE_METHOD_HANDLE->SET_KEY_ATTRIBUTE_NAME( 'MATNR' ).
wd_table1 ?= view->get_element( 'TAB_N' ).
WD_THIS->TABLE_METHOD_HANDLE ?= wd_table1->_METHOD_HANDLER.
WD_THIS->TABLE_METHOD_HANDLE->SET_KEY_ATTRIBUTE_NAME( 'MATNR' ).
In above code TAB_SEARCH is a parent Table so ion similar way take a new table view with your popin table name , in above example its TAB_N ..( this two names are from your webdynpro design name , where you bind your data source )
It will work :-) 