It's in this game, MechCommander Omnitech. It's one of several new 'Mechs the team has made, but as they're still revamping the logistics system, they haven't added it to the actual campaign purchase/mission files yet. The relevant data for the 'Mech is all there, though, and you can see it in the in-game Encyclopedia.
Just like retail MechCommander 2, the game saves are in relatively simple, plaintext files that can be easily edited without needing special tools. You can tweak pilot stats, give yourself more money, or give yourself otherwise-unavailable 'Mechs to play with pretty easily, once you understand how it works.
The easiest way to add a Mech would probably be to just swap one of the ones you have.
Say, complete the first Mission of Carver 5, save the game(for example 'game1'), then Alt-Tab out, find the file in the MechCommanderOmnitech\data\savegame folder(as above, it'd be 'game1.fit'), and open it in Notepad. At the bottom, you'll see:
...
[Inventory0]
st Chassis = "data\objects\jenner.csv"
st Variant = "Jenner JR7-D"
b Designer = TRUE
[Inventory1]
st Chassis = "data\objects\firestarter.csv"
st Variant = "Firestarter FS9-W"
b Designer = TRUE
[Inventory2]
st Chassis = "data\objects\werewolf.csv"
st Variant = "Razorback RZK-9C"
b Designer = TRUE
[IComponents]
[purComponents]
FITend
You may see an extra line('st Pilot =') in each 'Mech entry, saying which pilot is currently assigned to the 'Mech, but deleting it won't hurt anything.
Just change the entry under [Inventory0], to this:
...
[Inventory0]
st Chassis = "data\objects\bloodkite.csv"
st Variant = "Blood Kite Prime"
b Designer = TRUE
Save and close the file in Notepad, then re-open the save in MCO, and your Jenner will be replaced a Blood Kite.
If, instead, you wanted to add a whole new Mech without swapping for another, you'd have add 1 to an entry called InventoryCount up at the top.
[General]
l VariantCount = 0
l PilotCount = 108
l InventoryCount = 4
then add:
[Inventory3]
st Chassis = "data\objects\bloodkite.csv"
st Variant = "Blood Kite Prime"
b Designer = TRUE
just under the [Inventory2] Mech at the bottom.