UPDATE:  I think I figured out a way to do it.  Print the letter as a PDF then on esignature request hit Browse and find it.  It adds it to the bottom and you can go from there.  You have to add the Sign and date yellow tabs where they go. It is time consuming but I am using one of my clients as a test subject to see if it works.  And hopefully when I get the tax return done I won't get charged for another esignature. 

Three years later and I guess this still isn't a thing we can do.  I want to send Engagement letters to be signed BEFORE I work on their taxes.  I just tried and I can choose it (in ProSeries Pro), it puts a green check mark by it but adding it below or hitting next is greyed out.  I tried all the other forms they have on the dropdown and none work.  Anyone know of a fix or am I missing something?