Of course.  Remember, we live in an age where documents can be stored on a computer, altered, and then printed.  What you are handed by a buyer or seller may look like a form, but it may, in fact, be a document written by the other party's attorney, with many clauses tilting in their favor.  That is why I always recommend having an attorney review the Purchase and Sale Agreement or an Offer to Purchase (regardless of whether you are the buyer or the seller), BEFORE you sign.

But to answer your question directly, the Purchase and Sale Agreement is a contract.  Like any contract, if you can get the other party to agree to the clause you desire (or if they fail to read the contract before signing) you will have the language you desire or need.  (Of course, some language, like the provisions dealing with smoke and carbon monoxide inspection, must remain.) If you are, in fact, using a form provided by the real estate agent, there is usually an area toward the end of the document where you can write in additional terms.  Similarly, you can alter a term with a pen and get the other party to initial the change.  For more general information, visit our Massachusetts Real Estate Law Discussion Forum. Good luck.