Use <space>e<enter> to edit a file in your $EDITOR.
You can also overwrite broot’s behavior for enter by setting a verb in the config.
I wrote a simple integration to select and open a file from broot from within kakoune using “suspend and resume” if anyone is interested (works only with tmux in this specific implementation and makes assumptions about config locations; dirty but works 
).
EDIT: That was already discussed here, sorry: Broot: A new way to see and navigate directory trees - #9 by sgraf812