Hello, z.e.k.i
File: CIV4UnitInfos.xml
Not without changing the <Domain> tag to DOMAIN_SEA. But that would make these units boats. They would not work on land anymore.
If what you are after is an early game collateral-damage ship, you could for instance upgrade the (almost) useless Ironclad to one by changing the tags <iCollateralDamage>, <iCollateralDamageLimit> and <iCollateralDamageMaxUnits> to something other than zero.
There is nothing stopping you from making any ship into a collateral-damage unit. Depends on what you consider fun. By all means, make the Trireme a collateral-damage monster if that is what you want. Probably not fun in the long run, but for a game or two it could be.
As far as I can remember, the only ships that can do collateral damage are Battleships and Missile Cruisers. But those are both late game.