Can they paint some dental sealant on top of a dental filling to make the filling lasts longer?

No: In most cases it is not wear that causes failure but decay and the opening of the edges around the border of a filling. If you just added something on top of a filling you would have to cut away part of the opposing tooth to make room for what you addded. If we had a stronger material to put on top of the filling we would just make the filling out of that material. Nice thought but won't work.

Not at this time: Who knows. With the explosion in dental technology, these procedures may be the treatment of the future. But not at this time as far as i know.

Filling ; Sealants: Both are typically made of plastic resins. Often the sealants are "un filled" resins that are basically un-reinforced filling material. Dental researchers are constantly working on more durable restorative materials.

That depends: If the filling is an amalgam (silver) filling then no. If the filling is a composite (white) filling then possibly. If the margins of a composite filling are wearing, but the filling is intact otherwise then a sealant can be placed around the margin to prolong the life of he filling. If the filling is failing because of new decay, or it is cracked then this will not work.De.