Sure it's possible to open any XDG-compliant desktop entry file with any text editor and manually edit it inserting any icon path/name, changing options, etc. but it may end with errors so as soon as main program for opening such kind of files is exactly file manager, the file manager (and nothing else) should manage them as well. AFAIK only one of file managers cannot do it yet and it is PCManFM. What it is required for that?
1) icon in 'Common' tab should be clickable and it should open dialog for select icon from common categories or from custom file;
2) additional tab ('Run') in file properties dialog;
3) tab 'Run' should contain fields for editing 'Exec', 'GenericName', and 'Comment' properties as well as checkboxes for 'StartupNotify' and 'Terminal' properties;
4) probably 'Path' and 'Type' properties should be handled too.