Its actually very easy, I had to do it for my wedding venue, I had to make it easy to swap between Sanke and Ball lock so when we left the venue we could get it all back the way it was.
You simply need a Male and female flare fitting.
Not sure what type of fittings you already have but the basic Chain is this
3/16 or 1/4" Barbed fitting to 1/4" Swivel Nut(Female)
This goes into the end of your beer line, with the swivel on the outside.
Now connecting ball lock connectors is easy, they all have Male 1/4" flare connectors that connect directly to this. If you want ball lock, just screw it in.
For the Sanke side you need a 1/4" MFL(Male Flare)tail piece fitting like this
That goes in the also shown beer nut which you should probably already have from your Sanke hookup assuming it didnt come all as one piece
From there you have a male fitting as well, just unscrew the ball lock, and screw it onto the sanke connector. You'll end up with something like this
All of these will probably be ~10-15 bucks per line, so $20-30 for beer + gas.