Not DIY, but I wouldn't. The soft solder melts a lot lower temperature than silver solder, you might melt other soft solder joints on the line getting it too hot. Use whatever you're supposed to use to fix it. Fix it right, or potentially pay later.
clean the joint as well as you can and use staysilv paste flux. If you can get the joint apart and remove most of the solder first it will work a lot better. We normally cut the solder joint out and replace the fitting as stated in the last post this could be a future leak.