If you go to the server tab, then select 'Server Console'. If you enter text in the 'Domino Command' field then try to insert text into the exiting text that's there from the paste buffer, it clears the text that is in the field and pastes what you have in the paste buffer.
For example, if you have executed a command like
load convert -u LWSupportTrack.nsf * mail10.ntf
then want to say change the target file to, say, 'JamesJones.nsf', if you copy 'JamesJones.nsf' in the paste buffer, remove 'LWSupportTrack.nsf' from the command field, then paste 'JamesJones.nsf', the entire line disappears and all that's in the Domino Command line is 'JamesJones.nsf'.
I always, then, have to put the entire command into a text editor, modify the target file, then copy/paste the command into the Domino Command field. It should be possible to simply insert text into that field as needed.