Inserting blank lines into RTF file with AppleScript
You can use this Cocoa-AppleScript:

-- Cocoa-AppleScript-- you need these lines in your script
use framework "Foundation"
use scripting additions

on insertTextInRTFFile(f) -- the parameter must be the posix path of an existing file, example  --&gt; "/Users/someName/Desktop/xyz.rtf"
    set myText to linefeed -- the text to insert in the rtf (a blank line as example)
    tell current application
        set fileURL to its (NSURL's fileURLWithPath:f)
        set myOption to its (NSDictionary's alloc()'s initWithObjectsAndKeys_(its NSRTFTextDocumentType, its NSDocumentTypeDocumentOption, missing value))
        set {attrString, docAttrib, err} to its ((NSMutableAttributedString's alloc())'s initWithURL:fileURL options:myOption documentAttributes:(reference) |error|:(reference)) -- get the attributed string from the RTF file
        if attrString is missing value then error err's localizedDescription() as text -- not a RTF file or not a valid path

        -- it use the string in the 'myText' variable to create an attributed string 
        set firstline to its ((NSAttributedString's alloc())'s initWithString:myText attributes:(attrString's attributesAtIndex:0 effectiveRange:(missing value))) --  create an attributed string with the style of the first character
        set lastline to its ((NSAttributedString's alloc())'s initWithString:myText attributes:(attrString's attributesAtIndex:((attrString's |length|()) - 1) effectiveRange:(missing value))) --  create an attributed string with the style of the last character
    end tell
    attrString's insertAttributedString:firstline atIndex:0 -- insert an attributed string (blank line) at the top
    attrString's appendAttributedString:lastline -- insert an attributed string (blank line) at bottom
    set rtfData to attrString's RTFFromRange:{0, attrString's |length|()} documentAttributes:docAttrib -- create the data from an attributed string
    rtfData's writeToFile:f atomically:true -- write the data to the RTF file
end insertTextInRTFFile


After your script will have written to the RTF file:
 just call the insertTextInRTFFile() handler with the posix path of the RTF file as parameter.