FROM HUMMINGBIRD You will be able to incorporate the Helix 10 Chirp Mega SI GPS G2N into your current set. You will need to purchase an Ethernet cable, a dongle and possibly a 5 port switch. The Ethernet cables come in a variety of sizes. I have included a link for each of these products for your convenience.
Does your ipilot have the link? If so, then you will need the 5 Port hub and cable. If it is just ipilot without the link you will not be able to connect it so you won't need the hub or cable. Unless you will have more than 2 fish finders
You will not be able to network the mega or chirp to the 798 or 999. You can share the fixed frequencies such as 83/200 and the 455/800 but you would have to turn chirp off on the Helix mega unit. If you want to network all three units together you will need the 5 port switch and Ethernet cables from each unit to the switch along with Ethernet cable adapters in which you may already have 2 of