Pythia requires a block called 'QNUMBERS' which contains few quantum numbers like electric charge, spin, color rep., etc for BSM particles in the param card (equivalently the SPheno.spc.model_name), but in my case, there is no Block QNUMBERS in the spheno.spc file. Is there any flag to write these blocks in the spc file? If not, then what should I do to write these blocks in the spc file?

This is an issue I've come across too, and solved it indeed by adding the appropriate QNUMBERS blocks myself. You can copy and paste them from the blank param_card.dat that madgraph generates when you generate a process. Also, you should comment out the MODSEL block, otherwise pythia will just think you have the MSSM and set some default masses ...