Not possible.
The F-16 & F/A-18 mods rely on a simple model swap (a renaming of .cmd or .lom files).
There is NO Boeing 747 within lockon to represent a VC-25.
To make a decient model with LOD's, full animations of working parts etc would at the very least take a few months. No real point in spending such time on a flying bus just yet, when a lot of combat aircraft need either better models made (like the F-16 etc) or to be implemented totally (Tornado ADV, etc)
I have 1.12a Flaming cliffs with NO Mods installed yet after a fresh install.
The S-3's ability as a tanker has been in Lock-On from the start (1.0) and indeed Flanker 2.
The F-111 entry within the "MeInit.xml" is left-over code from Flanker 2.0.
While this entry is there (As well as the MiG-29K entry), it does not have an aircraft model. Therefore is disabled.
There are a few mods around that add the Flanker 2.0 models of the MiG-29K and F-111 to lock-On (1.02 or 1.1) , or you can add them manually if you have Flanker 2.0.
My word, 9 days and still no answer worth anything.
Well MrCholmondley-Warner can fix that for you, old bean.
The S-3 tanker that you require for your mission is already within Lock-On already.
With-in the editor, simply choose "USA" as your side, Set task to "Refueling" and choose type of aircraft as "S-3B".
Bob's your uncle, me olde gvn'r. Now you're cooking with gas.