Can i apply dental sealants on molars that already have filled cavities?

No: Sealants are bonded to tooth enamel and have poor bonding properties to existing fillings. So it would not be a good idea to seal an existing filling. There is however such a thing we call a preventive resin restoration in which we drill out the decay, place a composite resin filling, and then we use sealant on top of that. But that only works if you do the resin and the sealant the same day.

No: Not unless the area does not have a filling that will come in contact with he sealant.

Possibly: If the filling is a composite (bonded) filling then you can as long as there isn't any new decay. I routinely apply sealant as the final layer when i place a new composite filling on a molar. If the existing filling is an amalgam (silver) filling then no you cannot, as the sealant will not bond to the amalgam.

Maybe: Maybe. Depends on circumstances. Ask your DDS. See DDS with any pain which may be due to decay.

Sealants: Sealants do not treat cavities. They are used to cover over deep grooves on chewing surfaces of back teeth to prevent bacterial invasion. The material has to be applied by a Dentist, it's not a DIY project. Covering over existing decay will not stop the infection from spreading. Please see your Dentist.