Did you not read what Thor the Bassist said? It cannot be done! He states it in the very first line!
And anyway, if I can remember, experience is tracked per individual soldier and the chevrons you see is the unit average. If you have half strength unit of 2 chevrons, and retrain them so they have a full strength, the average will be 1 chevron in the entire unit - Half will be 2 chevrons, the other half will be 0 chevrons.

But if you must you could also use:
Code:
create_unit "<location>" "<unit name>" <amount>,<chevrons>,<armour upg.>,<weapon upg.>
Which creates any unit you want. So if you want five 9chev hastati with gold armour and weapon upgrades spawned in Rome, you'd use this:
Code:
create_unit "Rome" "roman hastati" 5,9,3,3