You could remove all the bombs on the A10, then add all but one of them back.
The command "addMagazine" won't allow such things, it just adds a full set of bombs to the Thunderbolt.
So, the best way would be compelingthe Thunderbolt to fire and then catch it's bomb:
_A10 fire "LaserGuidedBomb"
Of course, you'll have to add the A10 an eventHandler "fired" to detect and catch the bomb:
this addEventHandler ["fired",{LGB=nearestObject [this, "LaserGuidedBomb"; LGB exec "MyScript.sqs"}]
Notice that you must have the command "nearestObject" in the "addEventHandler"'s command line, otherwise, if you call it trhough a script, it might not be fast enough to detect the bomb (well, it usualy happens with bullets ;D so, with bombs you shouldn't have any probs, eheh!).