Can a collection agency add penalty or interest charges to an old credit card account that has been charged off?

When you say it was "charged off", what does it mean ? Was it discharged via bankruptcy proceedings or the creditor had advised you that it was not pursuing the debt ?

The interest and penalties will continue to accrue according to the terms of the loan service agreement. If there is a judgment, it will accrue post-judgment interest at the rate for your state. You can try settling with the creditor, paying the judgment in full, or see if bankruptcy is an option by getting a consultation with a bankruptcy attorney. You might also want to explore whether this debt is beyond the statute of limitations and is therefore uncollectible.

This is a very complicated question. As to interest the answer is maybe. It depends on your state laws, the original agreement etc. As to fees, probably not. The issue you are asking about is one being litigated often. It involves the intersection of the FDCPA and the TILA. I suggest you consult with a consumer rights attorney.