My first reaction to your issue is, why not if you are good at it. If you have the expertise to write marketing pitches, why not, you can write personalized welcome letters to your new PSAs using information from their Affiliate Profiles. However, for very many of us affiliates who do rely heavily on SFI to lead us by the hand, it is advisable to stick to SFI welcome letters. They are good, effective and can be easily adapted by any affiliate.
I have no doubt that SFI has tested these letters over time before they decided to let affiliates use them. No harm though if you can write your own. In this business, it is always best to use what has been tested and works well. If a little innovation on your part can help further with good results, by all means go ahead and do so. Really, it is results that count more than anything else here and that is what you should always aim at.