I know this question was originally from March and for TY19 - but I had the same question for TY20, and I think I found a work around. 

dd4vols method works if you want to replace the "Standard Client Letter" but not if you simply want to ADD one.  For instance, I have new clients sign my privacy policy before I get any documents.  Perhaps they just want me to review their tax return, perhaps they want an estimate on the prep cost, etc.  So, when I send the organizer (or letters printed from the 1040 page), I want to send the actual engagement letter to prepare the tax return, but don't want to send another privacy policy. 

So, from the Organizer, I want to have two standard letters - one which is the "letter" part with the engagement letter for prep work, and the other is the "letter", the EL, AND the privacy policy.  But as installed (by default), there is only one client standard letter that can be printed from the Organizers. 

However... I found a work around (I take no responsibility if you edit things and they stop working). 

Here is what I did:

1. Close ProSeries
2. Find the folder / directory where letters are stored on your hard drive.  Mine were in C:\Intuit\ProWin20\Common
3. Make a copy of all files that you may be editing (best to make a backup copy of the entire common folder somewhere else just in case)
4. Create a "template" for the "New_Letter".  For instance, the standard letter from the organizer (filename is ogmltr) may be a good starting point.  So you will want to copy that letter. 
   1. Copy template.ltr to new_letter.ltr (i.e. make a copy of ogmltr.ltr and name it newogmltr.ltr)
   2. Copy template.rtf to new_letter.rtf (i.e. make a copy of ogmltr.rtf and name it newogmltr.rtf)
5. Edit (using notepad or Wordpad) the letter "list" (so the list of standard client letters).  For 1040 letters, edit fdiltr.dat; for Organizer letters, edit ogmltr.dat; for Fiduciary letters, edit fifclltr.dat; etc
   1. Add a line with "Name of the New Client letter" comma "new_letter" (i.e. in the ogmltr.dat file add a line
      New Client Organizer Letter,newogmltr
6. Open ProSeries
7. Open up a client file
8. Edit the new_letter by going to "Tools" "Letters" "Manage Standard Letters" and you should see the new letter name (i.e. New Client Organizer Letter).  Select that one and "edit"
9. Edit the New_Letter by double checking all of the codes, making changes, etc.  Remember the codes for the Organizer and for the actual 1040 are different so if you used the 1040 letter as a template for your organizer letter, you will need to change all of those codes.
10. Save and Exit Letter

By doing this, I can print from the organizer panel a letter with the "standard instructions", a separate "engagement letter" and a separate "privacy policy" all from the organizer section (whereas by default there is only one letter).