Yep that's a K valve on the yellow tank. I get my fills the same way, however my setup is a Joe B CF tank with a DIN 300 valve with his screw in 1/8 inch male foster fitting. To fill my tank I must remove his adapter with the 1/8 male foster fitting and replace it with a DIN 300 to SCBA adapter I got from R&L, a nice fitting. That fitting terminates in a SCBA (male) fitting that hooks to my FD charging system.
That fitting on the FD tank does look like a foster type fitting, but the 1/8 fosters on my Marauder and my CF tank look distinctly different. Looks like its homework time for you unless someone else here can be of assistance. You are going to have to get one of those male foster like fittings that is on the FD tank to hook to their system for hpa. Once you have that you can work back to your tank.
You are going to have to work back from the FD fitting to your K valve Scuba fitting. Word of advise, be sure you want to continue using that relatively low pressure tank before you start spending $ on a string of adapters. Consider getting a used SCBA 4500 psi fire dept tank with valve off ebay etc. and selling your scuba tank. You will get so many more fills from a 4500 tank than your 3000 psi tank. Shopping for used FD trade ins is a whole new topic. Then all you would need is one of the mystery FD foster like fittings and a SCBA (male) adapter to 1/8 foster (male) fitting to get your tank filled. Once your ebay FD tank is filled remove the mystery FD fitting and screw in a SCBA (male) to 1/8 foster (male) fitting and there you go.
You might edit your post by adding a pic of your current fill setup and any hoses and adapters you have on hand. With that info there might be a short cut using your fill hose to make a simple adapter without breaking the bank on pretty polished fittings.
I suggest you go to several air supply sites, site sponsors first of course, and see what's available. You need to study up on the different valves K, DIN 300, and SCBA.
