NOTE: I made this on a game that includes the altered sprites from the Miranda Class (In the thread called 'Help with Importing ships'), I am not sure if it works without those altered sprite files.

//*********************************************************************************************************
Open 'all_cap_veteran' (C:\Program Files\Activision\Star Trek Armada II Fleet Operations 3.0\Data\odf\system) and add at the end:

fleetcapclassXX = "fed_excaliburr6"
fleetcapclassXXcapvalue = 5

Replacing XX with the number 1 higher than the ship before it.

//*********************************************************************************************************
Open 'all_veteran9' (C:\Program Files\Activision\Star Trek Armada II Fleet Operations 3.0\Data\odf\system) and find:

wpnname = "Promote to Veteran"

add above it

replacementXXcondition = "fed_excaliburr5"
replacementXXclass0 = "fed_excaliburr6"

Replacing XX with the number 1 higher than the ship before it.

//*********************************************************************************************************
Open 'all_cap_mixedtech' (C:\Program Files\Activision\Star Trek Armada II Fleet Operations 3.0\Data\odf\system) and find:

maxcap = 20

add above it

fleetcapclassXX = "fed_excalibur"
fleetcapclassXXcapvalue = 12
fleetcapclassXX = "fed_excaliburr2"
fleetcapclassXXcapvalue = 12
fleetcapclassXX = "fed_excaliburr3"
fleetcapclassXXcapvalue = 12
fleetcapclassXX = "fed_excaliburr4"
fleetcapclassXXcapvalue = 12
fleetcapclassXX = "fed_excaliburr5"
fleetcapclassXXcapvalue = 12

Replacing XX with the number 1 higher than the ship before it.