In Florida we have two acceptable Offers to Purchase - one is a FAR contract (Florida Association of Realtors) and the other is the FAR-BAR (a joint effort between the Florida Association of Realtors and the Florida Bar Association. Each comes with a variety of prewritten addendums - check the letter on the contract, and attach the appropriate initialled addendum. So far, so good. Sometimes, though, these addendums don't cover a specific situation. We may be fortunate enough to have a company list of addendums, approved by their attorney, or our company may have an attorney on staff. That's not the case for many of us.
Be wary - if you write your own clause or addendum, you're practicing law. In addition to that, you may be liable to one or both parties to the transaction if the transaction isn't happy for both sides.
Recently I had a verbal agreement between both sides, and was not comfortable writing it, so sent a summary of the verbal agreement over to an attorney, gladly willing to pay his fee to keep myself out of trouble. After he reviewed it, he said - even if I wrote a 30 page addendum, there isn't a good and clear way to write this, and the party most likely to be stuck is YOU. Upon his advice, we renegotiated the item according to his suggestion, and then the clause was easily written.
Just a reminder not to be in so much of a hurry that you do the quickest thing, rather than the best.