You can add to a statement, but keep in mind that remembering details after the statement does bring up some credibility issues. If it is the name of a witness or a doctor or a street that is one thing, but if it changes how the accident happened or who was in your car, that is something totally different.
This is precisely the reason why it is rarely a good idea to provide a recorded statement to an insurer. There is rarely any benefit and most often than not only downside. There is no formal mechanism for changing a statement and any changes you seek to make (if substantive about liability) will call your credibility into question.
In order to answer your question, I need a few more details. Is the insurance company that you gave the recorded statement to your insurance company, an insurance company for the car you were driving, or the other driver's insurer? Also, were you injured? Are you making a claim, or are you a witness or potential defendant? These factors determine how to best proceed.
Then, request in writing a copy of the transcript of your recorded statement and review exactly what you said. In any case, only change significant mistatements in your answers not minor ones. You may want to have a Massachusetts personal injury attorney review the statement in the context that it was given before you do anything.
There are more considerations that are fact-specific but this should get you started in deciding what to do or not do.
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It is not sworn testimony so it can be changed or completed. A lawyer might be helpful in forwarding additional details which you now recall and may not have been eplicity requested.
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