Well to get a DLC you will need PSN access which you can't get on your PS3 since you have 3.55 CFW on it. If you can find a DLC in .pkg form and it is 3.55 FW or below, it should work. But currently there is no way to get new DLCs working on a PS3 with 3.55 CFW.
This is one of the biggest challenges still. DLC is only available via PSN. If one can isolate the installation file just before it installs, then it can possibly be converted to a PKG. But first you need a hacked PS3 going onto PSN to get it.
Then there is a risk of banning.the other option is to convince the manufacturer of the game to release it to you, but I have a better chance to get pregnant than them going over $ony's head. The last option is to get a $ony employee to leak it, but that is theft and this risk is way too high. Your best bet is to do like so many others - get 2nd PS3