You might be able to use sodium sulfide. I once replaced a chloro on 4-chloropyridine with -SH using sodium sulfide, but the details of the reaction escape me right now since I did it a real long time ago. I will look it up in my notebook tomorrow at work.
I actually did it on 4-chloropyridine N-oxide, so I don't know if it will work on your system or not since the N-oxide strongly activates the 4 position, but you can give it a shot.
15 g of the N-oxide and 18.6 g of Na2S were dissolved in 1000 mL of ethanol and were refluxed for 1 week. The solution was then filtered, rotavaped down, and washed with ether. NMR confirmed thiolation. Yields were difficult to determine because of the presence of inorganic salts.
If you do try it, do it on a small scale first. Na2S and all of the H2S it gives off are pretty toxic.
Also consider the rest of your molecule, Na2S in solution will turn the solution very alkaline, so the rest of your molecule must be stable under basic conditions for the reaction to work.