Is it at all possible to edit transfer add-ons, such as monthly installments, sell on fee % and so on after the transfer has already been completed?

I can not find this option anywhere.  

not once completed I stand to be corrected but sure you cant

 

you can amend details when transfer is under offer

To do it you need to make a later transf.
If you try it with negotiation status you only can edit the mass value of transfer, not clauses.
 

Basically you only can do it if the transfer are show in future transfer.