@Pacco,
Welcome to the JMP Community. Let me try and explain this and solve the problem. The Add Rows messge only specifies the # of rows and where to add so JMP is executing what the function is telling it to do. In order to scale it for your needs , we can try the following :
dt = Open( "$SAMPLE_DATA/Cities.jmp" );
dt << Select Randomly(0.1); // Selecting rows randomly
SR = dt << Get Selected Rows; // Get list of selected rows
for(i = 1 , i <= N Rows(SR), i++,
dt << Add Rows(1,SR[i]); // Add 1 row after each selected row
);
Hope this helps and addresses your question
Best
Uday